Bastian:
How can I start writing a novel? I have an idea, I have some characters and the story but I have no idea how to start and where to go.
First of all you write your novel on your own. The NaNoWriMo site just provides resources to track your progress and lots of "motivational" support. Begin writing in your favorite word processor or if you're brave, on pen & paper but word counting suuuucks in that regard.
When you're ready to get tracked go to
https://nanowrimo.org/ and create an account. It will likely funnel you right into where you need to go but if it doesn't at the top under the My Nano menu go to Projects -> Announce New Project. give it a name, associate it with NaNoWriMo 2020 (if you want the awards/badges) and give yourself a goal. The default was 50,000 but they've expanded it so you can set your own writing goals. You can do short stories, you can edit/rewrite etc.
It has always been primarily a tool to motivate budding writers to write and not any kind of formalized contest with strict rules and guidelines. 50,000 is the "default" but you can do it more or less as you so desire. I believe they also allow you to do pages for things like a script but don't quote me on that.
As for writing the novel that...that is trickier. Think about novels that you really enjoyed and emulate the overall structure. For example I often do a prologue that sets up the big bad conflict well before the protagonists are aware of it, then the next couple of chapters are introducing characters and setting before launching them on adventure.
One thing I am trying to incorporate is a more cinematic style. I watch a lot of "film analysis" videos and one of them on the movie Inception talked about how Inception did such an excellent job at trimming the fat, that every scene is impactful. So many movies waste time...for example in another movie if a good guy has to infiltrate a building a movie might spend 20 minutes detailing how they steal a uniform and then an ID and then have to navigate this obstacle or that which is fine if the heist is THE purpose of the movie but for a movie like Inception...the heroes just do it. The movie just cuts to them being in position. It had already established that these were the best of the best and so it kept itself laser focused on the things that were important and didn't get sidetracked.
In writing it is common because you don't have the time constraints with movies to get a little...meandery. I think back to Lord of the Rings and there are so many little side adventures that don't really do much for the overall story. I love Tom Bombadill but what value does that whole thing actually add? It was what...3-4 chapters just to get them a couple magic swords?
Anyway even though it is a full length novel a design goal that I have is to trim the fact. I want this thing to be action packed with tight focus as if it was a movie.
Another bit of advice is don't worry about your first draft having cohesive themes or characterization or plotlines. It is the second draft that you go back and make it seem like you knew what you were doing all along :D