So, the whole idea behind NaNoWriMo is to get you writing every day and focusing on a deadline. The idea is to hit 50,000 words, because that is the length that most people consider a novel, even though most novels are much longer than that. Most works under 50k words are considered novellas. You don't get a prize or anything for finishing, but it's really nice to write a whole bunch and finish at the end of the month knowing you either have a first draft of a book, or at least the beginning of a first draft, done. Think of it as a challenge that you can decide to take on instead of a contest.
NaNo has some "rules," but a lot of people ignore them and instead just want to use the event as a time where they can write and have a deadline. If you want to rewrite something you've already written, that's fine! No one is going to stop you. Originally NaNo was all about people who had never written a novel before finally actually writing instead of just talking about it. But I think NaNo can be a great time for rewrites, especially if you redo your entire novel. The only real rule you have to follow is that you have to try to hit 50,000 words by November 30th.
Any genre is definitely okay. You can do pretty much whatever you want to do. People have done fantasy, romance, historical fiction, sci-fi - it's totally up to you. I have even heard of people writing nonfiction, but I think that's harder.
To participate in NaNo, you just have to sign up on the NaNoWriMo website by November 1st. Then, you follow the instructions to update your word count each day until you hit 50k before November 30th.
Hope this is helpful info!