I'm going to start DMing my first campaign in about 2 weeks and I was wondering if anyone had any tips? And not ones like "don't be afraid to make mistakes". Does anyone have any?

Any detail? E.g. campaign setting?

Honestly, don't worry. As a dm, the game and world revolves around you, your decisions, and how well you handle everything in between. Everyone has the jitters about that. It's a lot of responsibility. But really, you don't need to worry. You, same as the players, are a player. Your fun matters. Always try to maximize fun for everyone and never forget you are one of those 'everyone'.

You'll play with people you don't like, you might not always enjoy every player, and that's normal. Of you've got a close group of friends then even better. But just know, everyone has a different type of dnd they play. Your players will almost always be enjoying the game somewhat, and if they don't, there's a good chance they just don't mesh well with your game style. Don't be put off by it. Improve, invent, re run.

It's just a game, so have fun.

What Canimanamino said is really good advice. It's not just the players that matter, but yourself too. I also recommend to keep the game running. Don't get stuck too long on any one thing, keep the action flowing, and don't lose focus and go away on some random conversation. If the players start demanding things from you, don't be afraid to say no. 

Hey. If you have no idea what kind of campaign you want to play with players I recommend to check three campaigns in my signature.

Well, I'll be honest. no matter what you do right now, you will not feel prepared, and you probably will make mistakes for this first session. Failure is the greatest teacher

1. Set expectations with your players. Make sure they understand that as the DM, what you say goes. Your role is to a) tell/facilitate the story, b) be a rules arbiter, and c) keep the flow moving.
   1. Along these lines, don't be a jerk. I like to set the expectation that if there is a rules dispute I will make a call in the moment based on what we know and can easily look up, but if it is disruptive I will make a decision to ensure the flow of game continues and the rule can be looked up later.
2. Be flexible. Practice "yes, and ... " improvisational story-telling. Try to limit your "no" to disruptive things, rule breaking, game breaking, and fun compromising things.
   1. Remember, "yes, and ..." can be both fun story-telling elements that can move the story along, as well as consequences for players being stupid.
3. Don't railroad your story. Try to be reasonable when your players want to do something you don't expect or even don't want. If you set the expectation that the players move down a particular story, you can always refer to that as a reminder for them to consider some positive metagaming to get back on track with the story.
4. Take feedback with grace. If the players are not having fun, talk to them about how the game can be made more fun and see how you all can enjoy the collaborative story-telling that is DnD.
5. Let the players RP with each other (encourage it!). If they start talking to each other in-character that can really enhance the gameplay, and that is gameplay you can watch instead of having to drive any sort of plot.
6. Nat 1 is only an auto-fail for Attack, and Nat 20 is only an auto-succeed for Attack. Saving Throws and Ability Checks can still succeed on a 1, and fail on a 20, depending on the result and the DC you set.
7. Persuasion is not mind-control. I recommend making players know that no matter how good their Persuasion is, they will not be able to convince creatures to do something that is against their interests (a dragon hand over their horde, a shopkeep giving away free gear, a king abdicating their thrown, suicide, etc.).

Here are some of the things I was told when I started.

1. DM is god, a.k.a you are god. You have final say on what happens. On that note, your plot for the campaign is merely your best guess. The players might act in an entirely different way. Whe that happens, adapt.

2. Don't plan to far ahead. If you do this you will automatically start to speed up the campaign to reach that end goal. You lose a lot of interesting roleplay that way.

3. Sometimes, especially if you are at a lose because of something your player's did, you should have the player's actions come back to haunt them. This can help them learn to think before acting and, consequentially, make less stupid decisions.

Most of all, D&D is supposed to be a fun game. Don't try to hard and have fun!

could anyone give me some tips as a dm since I'm new to the game, I have a question can I use a character I made before be a npc