basically, I'm playing DnD with my one friend as the player (I've been DMing for about a month and a half) and I have a campaign that I make up on the fly then write down for when my sisters play (probably not gonna happen) or my other friends play. I just kinda want to talk about my campaign and get some advice maybe...?

Well, you're in the right place!

What have you been playing, and what sort of advice did you want to get? 

well it's just a campaign I made myself, and I was just wondering if someone could check it out and give me some tips, maybe point out some flaws...

Basically, the player starts in a forest, and they have to find their way to these big town gates. They roll for Strength to open them, and are then in the town.

Next, some random guy runs up to them and smacks them in the face, then runs away, being chased by guards. The area to the right is the shopping district, to the left, the housing, and directly in front is a large castle. Two NPCs from the town can be recruited, Joe and Alex. Joe is a blacksmith, Alex a shopkeep who knows a lot about magic items. You can buy a homebrew item from Alex, and it's pretty OP.

You can steal and train a German Shepherd from one of the houses. In the castle, after getting past all the guards, (hopefully) you will have to face the king, who happens to be a Lich. The random guy from earlier runs in and tells you about the bad guy, a person named Donovan Phoenix. If you ask the king about him, he'll tell you where he was last seen, and then banish you from the town (unless he kills you). Hopefully, you'll have Joe and Alex by this point. If you don't, you won't be able to recruit them, having been banished.

You walk onward until you reach some mountains. There's a cry for help. If you choose to follow it, you find Marcus, who is stuck in a hole. If you get him out, he will join your party and increase your survival odds in what I call "The Final Showdown", which is when you fight Donovan. 

You can continue walking to where Donovan was last seen, but will eventually get attacked by Donovan's hunting party. (hunting for you, not animals). There is a large group of them and two leaders. They have giant wolves with them. If alex is there, he dies in the fight, killed by the leaders. If he isn't, the leaders mention that they killed him. (this is important for later)

If you kill all of them or get away, you will be safe for a while. No matter what happens, it is impossible to kill the leaders in this fight. (also important for later)

Then you can keep walking and eventually see a Phoenix in the top of the trees. You can ignore it or follow it, but either way you will come across a place called Upside-Down Town. It is upside down, with the sky below it. if you fall, you will fall into a void and die. If you have the homebrew item mentioned earlier, you can activate it and it will save you. Upside down town is as far as we've gotten so far.

I know it's a bit much reading. Sorry...

This reads very much like one of those old Chose-Your-Own-Adventure books where you read a bit of story, picked yes or no, and got directed to another page to see what happened.  You did this time and again to see what happened in the end, and could play through multiple times to see the different endings.  Kind of fun, and I have seen computer games much like this that could get very complex.  The King's Quest series?  Maybe?  I don't remember.

How familiar with the D&D system are you?  5th Edition D&D is one of the simplest versions of a game with a long history, but it remains fairly complex for new players or DM's.  You mention homebrew magic items.  I have found the Homebrew creation tool and editor impenetrable.  I can't for the life of me figure out how to create an item and share it with my own characters.  Will you be using D&D Beyond for character generation?  Are you playing online or in person?  Have you got a Virtual Table Top in mind?  Do you have a subscription?  Have you access to any of the rulebooks?

You have only one player in mind.  D&D is a team game.  It is designed for multiple players, each with somewhat specialized roles, to work together to deal with problems they could not manage alone.  You make no mention of levels.  At one point you mention homebrew (and presumably standard) magic items for sale.  What power level is this game meant to be?  A level 20 would walk all over something like the Final Showdown, but I don't imagine a German Shepherd would be of any great use at that level of play.  At the levels a normal, non-magical dog would be useful, two leaders and multiple (that's at least 3 I'd guess) Giant Wolves would eat the party alive.  What are the levels of Alex, Joe, and Marcus?  What classes?  How well equipped are they?  They would have to be pretty powerful, at the risk of making the player feel unimportant.

The first actual fight you mention is the Final Showdown. That is a large group, with two leaders, and multiple Giant Wolves, (plus Donovan?  It is unclear if he's actually in that combat) against a total of 5 characters if you count the German Shepherd in.  I would not.  You will be running all but one of these, so you are spending almost all of your time rolling dice for a combat of you against yourself.  It does not appear that the decisions the player makes can ever improve anything.  They can lose out if they miss things, but no matter what, they cannot come out ahead.  If the player does not play their cards right, they could end up entirely alone in the Final Showdown.

You say that Alex dies no matter what, and that the two leaders cannot be killed due to Plot Armor.  It is unclear if Donovan can be killed or not.  The player is getting no say at all in how the story plays out.  They have less freedom than even those Choose-You-Own-Adventure books gives.

Dungeons & Dragons is largely fun because it allows players to enter a fantasy setting and be heroes.  It's all about making choices and seeing the consequences.  The players want "agency", they want to do what they do, and feel like part of an ongoing story rather than watching the plot go by like they are reading a book.  Each player, working together in a group, with the help of the Dungeon Master, collaborates to create an improvisational story where the players are the main characters. Think in terms of Lord Of The Rings where the player characters get to be the good guys.

My advice would be to play through some Adventures yourself first before you DM.  If you've already done so, great!  Welcome to being a DM.  :-)  Take one of the Adventures you have been through before and run that.  You already know what it's going to be like and can adjust it to suit your needs.  Doing a campaign of your own is a difficult art and takes a great deal of practice.

I see a few problems. What if the party fails the strength check to get in the town? Everything stops. The advice is: don’t put necessary things behind checks.

The rest, overall is a railroad. What if the player doesn’t even want to go to the town? What if they decide to chase down that guy who snacks them? Why is their only one dog in the whole town? Maybe they don’t like it, but like a different dog. Why are only two people “recruitable?” I guess that’s a bit more likely than the rest. Why would you give someone you just met a magic item? Why would you(the DM) give a PC an overpowered item?
What if they don’t want to go into the castle? What if they see a lich king and attack it? What if they don’t care about Donovan? What if they don’t want to go in the mountains?

What you have isn’t a campaign as much as a linear series of events (it’s more like you’re writing a story than playing a game), players will always mess them up and either ignore things or go in a different direction. Don’t assume your players will do what you want them to do. And don’t put in enemies that can’t be killed. And don’t decide beforehand that their allies will die — that’s what the dice are for.

Okay, let's go through this with some questions that I hope will help you to flesh out this adventure somewhat! Remember that these questions are for you to answer so that your adventure flows together well - if you get to a point where the questions can't be answered to get to your next plot, then you need to change some things!

My first question would be why are they trying to get to the town? The standard premise for starting this sort of thing (though certainly not the only way) is for the player/s to have a reason for being there. For example, I opened my campaign with the players all being drawn to a small village by a plea for help sent out by the town elders. The characters all met there, and I have them a very vague explanation that they heard the plea and came to help, leaving it to them to work out exactly why their character would be there - to be a hero, to get paid, curiosity, boredom, ulterior motives - the world's their oyster, but I'm telling them that they arrive to this town on this night to do this thing. So, why are they trying to get to the town?

This guy who runs up and smacks them in the face - what happens if the party chase him? And why did he smack them in the face? Who is he? Why does he later turn up (of all places) in the kings throne room?

Recruiting the NPCs - what are they being recruited for? why would they come with the party? It sounds like this homebrew magic item is necessary to get past the upside-down town, so it shouldn't be left to chance for them to get it or not!

Why are the adventurers trying to get to the throne room? why wouldn't the king kill them for doing so? Why would the NPC's recruited from the town go with them?

Pre-scripting that certain people survive a fight and others die is a difficult thing to make work. If a leader kills Alex, why wouldn't the players jump to attack the leader, and the leader then has to escape by some mysterious means. Better to keep the people who need to survive in the background - the leaders turn and ride off on horses the second the fight starts, for example, and don't get involved - than to try and pre-script that they survive. 

Overall my advice to you would be:

1: Don't try to write what will happen, write what has happened. Instead of "the players fight their way through the guards to see the lich king", say "The king is a reclusive Lich who is suspicious of anyone who wants to see him, and doesn't welcome visitors. He is always holed up in his throne room, high above the town, and he is the one who knows where to find Donovan". Now you've gone from telling the players what to do (fight the guards and see the king) to telling the players what they need to achieve (you need to get this information out of the king). Let the players decide how to do it - climb the outside of the tower, or fight through the guards, or get jobs as guards, or try to deliver a package for the king, or dress as jesters for his entertainment to get close to him - or whatever they decide to try. 

2: Avoid getting too caught up in the small details of things they could do - tell them there is a dog there, but don't spend too long with the possibility that they could train it - let them decide if they want to do that and then work out what to do (animal handling rolls, for example) to see if they succeed.

3: Give them a reason to do something, and let the story write itself. Your plot thus far reads a lot like the players didn't decide a huge amount of it. They should be given a reason - "the bandit king who regularly attacks the town lives in yonder mountains, oh brave adventurer, won't you go rend his slimy head from his sloped shoulders?" rather than "and now you journey to the mountains". Your job as the DM is never to tell the players what they are doing, only to be their senses (tell them what they see, hear, smell) and to tell them how their attempts to do things turns out.

So for your plot, I would say you need to first ask why all these things are happening - why do they go to see the king, why is the random crazy man popping up with information, why do they go to find Donovan, why is donovan after them, etc., and then use that to explain why things are the way they are before the players get involved. Then whatever happens when the players goof off isn't going against any sort of pre-planned plot.

I look forward to seeing what you can build out of this! I like the idea of upside-down town, but I wil lsay that if the plot goes there, don't give them any chance of not having the item that makes them not instantly fall into space!

Thank you, this really helped me out. I really appreciate it.

I worked out all of the kinks, so here are the answers, in order:

Technically, they aren't specifically trying to get to the town, but they will eventually come across it.

If the players try to chase the guy, they have to get past the guards. If they manage, the guy will tell them about Donovan Phoenix right then.

He had no reason to smack them in the face. For the game, it was an acknowledgement of his presence. To the players, it was for no reason. He is a wiseman known as Jim, though the players won't find out until The Final Showdown. He turns up in the throne room because he knows that someone needs to take out Donovan, and he deems the players as most worthy.

The NPCs are not being recruited for any specific reason, and don't have to be recruited, but it will make life easier. They would come because they were in search of adventure. Well, Joe was, and Alex was his friend so he comes too, if they do come.

The homebrew item is not necessary to get to Upside-Down Town. It also does not do ONLY that. It is a protective item. You can get to Upside-Down Town without it, but attempting to is risky. Upside-Down Town is not completely necessary, and the players can avoid it if they choose to.

I actually messed up the writing a little bit, but I meant to put in the fact that the random face-slapper suggests they do. They can, of course, ignore him, but will still hopefully go where they are hoped to. If not...I guess I might have to summon a tarrasque...

Hope I don't have to...

The King doesn't kill them because he knows they'll kill Donovan or die trying, and, believing it to be the second option, lets them do so.

The NPCs would come for the same reasons mentioned above.

Thanks for the questions, and the advice. I think the leaders will Misty Step away. Thanks a lot.

I actually messed up the writing a little bit, but I meant to put in the fact that the random face-slapper suggests they do. They can, of course, ignore him, but will still hopefully go where they are hoped to. If not...I guess I might have to summon a tarrasque...

So a guy runs up, slaps them in the face and shouts "you need to see the king", and this is their motivation to potentially fight their way into a castle? It seems quite unlikely to hook the players in. Summoning the Tarrasque is the equivalent of "rocks fall, you die", so I wouldn't recommend it!

This still doesn't explain why they go into the throne room! Why does Joe want them to go there, just so he can turn up and tell them about Donovan? He could have said something elsewhere!

Just keep asking "Why" until you get something that makes sense. If it doesn't make sense, change it!

The party is going into the town>Why?>because it's there. Ok, makes sense.

The party fights their way to the throne room>Why?>Because a guy slapped them and told them to.>Why?>Because he wanted to turn up afterwards and tell them to go and kill Donovan.>Why did he have to tell them in the throne room?> no idea.  Not so much sense there, I'm afraid.

Thanks