You did it! You decided to be a Dungeon Master! We are happy to have you join our ranks. However, now you need a campaign. Starting to plan one out can be difficult, especially if you've never done it before. So, let's talk about how to start planning one and what approaches are available. Starting can be one of the hardest parts so hopefully I'll be even a little bit of help. Feel free to ask questions as well.
Relax, You'll Be Fine
First thing first. If you are running a game for the first time, relax. It's not so scary. There is some responsibility and some preparation work that goes into it but it'll be fine. We all had to start at some time before getting where we are. Being a Dungeon Master is a very fun and in a different way than being a player.
Do You Know Your Players?
If you are starting off as a new Dungeon Master with a group you've played with for a while, you'll know quite a bit about your players and what kind of game they like to play. However, you might want to ask them some questions ahead of time if you plan to try something very different than they are used to. If you don't know your players at all, it might be worth having a session to just talk about what they want and taking care of the ground work. I find that with some of the people I play with, this can be a tall order. Getting everyone together in a physical place to take care of that can be tricky and might not seem worth the effort for your players, though they'd be willing to do it for an actual game. For this reason, this kind of stuff quite often ends up in a pre-game Skype call from what I've seen.
If you are playing with a brand new group, you should be asking questions a head of time. You should also be ready to go in a different direction if it turns out that what you had planned isn't being received as well as you thought it would. You can do that, and you should be prepared to. Even if it is being well received, it could go in a direction you didn't expect. This is also true for groups you know, but I find that new players are more surprising for me until I get a feel for them. That's not to say the ones I'm used to get boring and I can perfectly predict them, but I get more of a feel for them. However, the first few sessions of a campaign can be more fluid as you get things rolling.
There is a large amount of pre-made material available for use in tabletop role-playing games. Some will obviously have more material than others. However, one strategy to start a campaign is to go into the pre-made adventures or campaigns. It's a different set of skills than coming up with a campaign from scratch, but it provides a starting point. That starting point should in theory come from someone with more experience than you and so help you make your first campaign better. Even if you don't run it exactly as written (this is very common), it can work as a good starting point and result in encounters, characters, situations, and story that you wouldn't otherwise think of. I will say though that you should make sure to know what you want to do though. You aren't just running the adventure as written; you'll be bringing it to life and making it your own. This means you need to have a good feeling for it, even if you are changing things.There will also always be blanks in the adventure that you'll need to fill in. I find it's best to see it as a reference and inspiration instead of a script that needs to be followed.
You can start your own campaign by thinking about the sandbox it'll take place in. Building up the world to have its own interesting elements can help you come up with your own, hopefully unique, conflict. Different worlds help inspire, or at the very least reinforce, different kinds of stories because of their rules. It can also allow an interesting place for your players to go feeling around for what they want to go after. You can dangle multiple setups in front of them and just go after the one that gets a bite. This becomes a lot more enjoyable if the place that they are going through is interesting. You don't need to come up with the entire world at one time, but you should at least think about the place where things will start.
Flipping things, you can think of the rough outline of your big bad(s). Characters are an important part of stories and villains are often the best remembered ones of them all. Making their motivations, their means, and their power make sense goes a long way. It can also help you come up with completely different stories when you come up with a particular villain you've never thought of before. Villains and conflict can also be very inspirational. A certain kind of character, such as a classic vampire, might inspire a Gothic style location (oh, hello Raveloft). It could also inspire a different location by taking that classic style of villain and putting them in an unfamiliar situation. Your conflict could be not centred around one being either and instead be around a force of nature, a war featuring many different complex threads but a small number of goals at their centre, or something far more bizarre (as gods, hunting down piece of an item that was never meant to enter the mortal realm over centuries).
You can arrive at the same stories regardless of which of the above you choose. It's just a technique to help get you thinking and it's sometimes useful to change to the other to get the creative juices flowing and coming up with new campaign ideas. If you are having trouble coming up with an idea in the first place, trying one approach and then the other can help you get some ideas you can turn into a workable campaign after a couple more passes through the above.
Too Many Ideas
That's great! Write them down. You never know when it might come in handy. You might use one later for a subplot, or an alternate climax, a new villain, the next story, etc. There are a lot of uses for such a thing. Now, we need to pick one. Sometimes, it helps to just leave it for a bit and look at them afterwards. Sometimes you'll find yourself drawn to a couple while you are doing things and realize that you really want to run one of those. If it's just one, that's great! We got our idea. Otherwise, you might just need to sit down, look at what your players want, look at what you have, reduce to a smaller list and finally make a tough choice. This stuff will be quite high level so it's very unlikely you'll be dooming yourself here. However, the right idea can make things very easy going forward because it makes role-playing and improvisation for you a breeze.
When I have something that you want to run, I typically like to run it through a sanity check. This takes the form of looking at the situations and campaign as a player would and think about what options I would have. What I'm trying to do here isn't predict what my players will do, but make sure that the situations I have allow for multiple solutions. A situation on its own might be fine, but the next scenario might presume something about the previous. If I find something like this, I'll either change it so it doesn't or note that there will be a factor that influences the situation from before. Depending on how the rest of the campaign goes before that point, this factor may be different because of how my players dealt with it. If it's so important that I can't leave it as just a note, it's probably too railroady and I'll change it.
Other Useful Things I've Written
There are some other things you can read as well. Out of the things I've written I would recommend precedent, reducing difficulty through situation, degrees of influence, and designing a combat encounter. These are additional readings and you can be perfectly fine without them. You just might find them a bit helpful. You can always come back later if one of those elements sounds interesting. When starting out it's not good to get overwhelmed.