Here are just 4 things that you should know. This may be updated from time to time (if I think of anything that should be or needs to be added) so check back periodically.

Blank Breed
Please do not choose a breed for your character, because none of the warrior cats are one breed; they are all mixes, because they're wild. However, characters can carry some traits from a breed, like the small, pushed-in face of a British Shorthair or something.

Names
If it's a Clan cat, the name MUST be two-part. Example: Rainclaw. And remember, the second half of the name (in the example, it was claw) is NEVER capitilized. Wrong: RainClaw. Right: Rainclaw. Get it? Got it? Good. xD If your character is a rogue, loner, or kittypet, it can be two-part or one-part.

ONLY THE CLANS!
It's okay if your character was born in one Clan and then somehow ended up in another Clan and stayed there or something, but please don't say that it came from a tribe of foreign cats that live far away and it floated down a river to the Clans and got taken in! Only the Clans, people, only the Clans.

Aging
Time goes by in-roleplay exactly the same way time goes by out of roleplay. If a month passed, a month would've passed in the roleplay world, but not in one thread that is supposed to be in the time period of a day or something. And so if your kit character was 5 moons and a month went by they would be 6 moons, and would become an apprentice.