When you see a thread with [sign-ups] in the title, post your priority in there (p#, number depends on how many games you are in currently. 1 if in none, 2 if in one, etc). When the sign ups close, if you get in, you are sent your PM, a new thread is posted where the game is played, etc. If you don't get in, try again. There are always a bunch of people who don't get in, and the list is randomised anyways (except reserved spots, which are kinda... reserved. If you want one of these, get yourself friends who like to make games).
Look for beginner games/standard games WITHOUT experimental stuff (important!) if you aren't that experienced; you'll just get confused and mess up big time. The difficulty is in the title, and reading the OP usually makes it pretty clear if it's experimental or not. SOme poeple are even kind enough to state that the game is not fit for beginners (which most standards aren't)