like what do we have to know and how can we prepare?
You most likely have to get it cleared with your school before you do anything with other students on school grounds: the school is liable for anything you do there and they will disband you if you don't check to make sure it's okay first. The school should then have an order of conduct for you to go through before you start, it's likely to be something like finding a chaperone, as well as getting signatures from interested students. If you set the club up properly, you might even get school funding for it.
Put up flyers around your school is probably the best way. Organize a meeting with everybody that is interested and go from there. They have a Glee club at my sister's school but from what I've heard, it's a bit of a fail.