New DM, very indecisive about homebrew

I'm writing a homebrew as the first campaign I've ever DMd, I know it's very ambitious but it's something I'm excited to take on. I have a few months before we start playing but I have no idea what I want the actual plot of the campaign to be. It will go on for a few months with players who have either never played or only played once. I'm too indecisive about what it should be, as I don't want it to be too "basic" like slaying a dragon or something (no shame I just feel like it would get boring with our party too easily, and don't want to play too safe) , but am worried I won't be able to properly execute more intricate campaign ideas. Any advice or ideas on choosing a plot or suggestions for the campaign?

If you've never DM'ed before, it might be wise to start out with a pre-existing module. It's a lot of work being a DM even when running a pre-written module. Building a world on top of that is going to be an additional challenge. Get a feel for being in the chair first, and move on to homebrew after you're comfortable.

For a first campaign and a first-time DM and inexperienced players, you should keep it simple. You might not even need a plot, per se.

Put the party in a town at the edge of the civilized world. Then you have a built-in conflict between the townsfolk and the monsters, bandits and cultists out there in the wilderness. 

You could go with a monster of the week to start. Maybe there’s an adventurers guild, maybe the mayor hires them, maybe they grew up there and a friend is in trouble. They go out into the wilderness, kill the bad guys, go back home. Or maybe not always kill. Maybe they negotiate with one group for protection in a certain area. Throw in progressively harder bad guys. Throw in distractions — they were sent to deal with the bandits, but there’s a mysterious tower over that way no one mentioned  

Play a few sessions like that. That can help with lots of stuff. For one, it will give you and the players a chance to figure out the rules a bit. It can also give you some time to suss out what kinds of things people like or don’t like so you can start to bend the campaign in the direction that people enjoy. 
Then at some point, you tie it together. Turns out done if the monsters/bandits/cultists have been working for some overarching person or group. Probably whichever one stuck in the party’s head. They find the person, kill the person, or otherwise foil their plans. Ta-da, campaign over. 
Now you have a bit more history, you’ve filled in some of the blank spaces on the map, and you can let the PC’s actions shape the world state for the next campaign. 

I am in the camp of the beginner DM needs some non homebrew adventures to start with.

My reasoning has nothing to do with the homebrew, but the actual detail of DMing an adventure.  You learn how to pace.  DMing is different from playing so what you need at your fingertips is different.  You can learn to actually DM with out worry or having the story get in the way.   Your homebrew will be there later.  Have patience and learn to actually control the session with out appearing to control it.  

As an first-time DM either use a prewritten module, or base it off of an existing story - e.g. Robin Hood & his Merry Men, or King Arthur and the Round Table, etc...

I also am preparing my first ever one-shot based on a classic short children’s story. With some help from the community, I discovered a monster that I was able to reflavor to be the antagonist. With that in mind, I have a few monster statblocks for the main npc’s should the party decide to turn on them for whatever reason. I’d like to think they won’t, but it’s best to be prepared. 

The thing is, no matter how much you prepare, things will happen that you simply cannot foresee. Thus, it is usually better to just get practice in as well as watch how people run games before you do so yourself, hence why people recommend using premade material to practice with rather than homebrew. 

Hopefully giving you a glimpse of what is going through my mind can help you with your adventure. Feel free to PM me if you need your bounce any ideas off me or have any more questions. Good luck!

Folks on these forums love to suggest "you should start with a premade campaign" to every new DM.  I am not really fond of that advice - the best thing a DM can do for their campaign is to enjoy the content they are running.  If the DM is not having fun, the game as a whole suffers.  To that end, if the DM finds enjoyment in making their own content, that is a far greater boon to the campaign than the ease of prep that comes from running a premade campaign. 

Since you do want to do a homebrew, I think you should try not overthinking it.  For someone new to homebrew, the easiest thing you can do is start with a basic core idea, even if it is cliche.  Once you have that, you can use your players' backstories to build on the core and flesh it in the directions the players telegraphed they want to explore.  Now you have something that you were interested in (the core element) and have transformed it into something a bit less cliche and geared toward your specific group.

I will also say that you should not overthink whether your players will like the game or not.  You are a new DM and they are your friends - they should understand that there will be hiccups and a learning curve.  At its heart, what makes D&D fun is telling a story with your friends - and as long as you all remember that, even a mediocre story of a beginner DM can still lead to enjoyment. 

Personally, I am DMing my first homberew campaign, and I just made a quick list of dungeons missions for my players to go through, and then get more upgrades rogue like style, and then started working on character arcs.  

If you want to make the first campaign you're ever going to run from scratch, it's okay to keep it basic. You don't have to hit it out of the park every time you sit down at a table to write something up. Here's a couple of ideas you could flesh out:

• Border town. The players are trying to make money to buy tickets on an airship to fly out because the town is isolated in a toxic bog. Cultists have infiltrated the village and want to summon a minor demon lord. As the players try to fend off mutated beasts for gold, the cultists turn the villagers against them to create hate and mistrust (food for the demon), and the players finally confront the evil at the climax of the summoning ritual.
• Desert. The players raid tombs and accidentally unleash an evil curse. They have to defeat an aspect of a long-slumbering god of diseases and scarabs before it's forces overrun the towns of the desert and the god fully awakens (first couple of sessions they just dungeon crawl and gain a few levels, then they unleash the curse and must arm the towns and weather the storm)
•  Jungle. The players have been enlisted by the queen to journey into the Eternal Forest, find the Temple of the Serpent's Fang, and recover a powerful artifact to cure the king of an insidious poison turning them to stone. They are attacked by rapid forest beasts (jaguars and the like) before they finally enter the temple to find it's Yuan-ti guardians slain and the artifact gone. They return, having failed, only to find that the king was actually fine and he just used the players to find the temple for him, and he transforms into a hideous beast using the artifact to slay the characters.

Keep in mind that these ideas could use a fair bit of work and expanding upon, as I came up with them from the top of my head. Feel free to go in your own direction, these are just ideas, use them however you want.

I've also noticed a lot of the people on here are saying to run a prewritten module. That is definitely one way to go, but don't feel pressured out of using your creativity. There are many ways to play DnD. If you do choose to use a prewritten module, I would recommend The Lost Mines of Phandelver or Dragons of Stormwreck Shore. Both are excellent modules to use for a first time DM, and I have played both and DM my first game on Phandelver.