I've added some screenwriting links to the right so here's a little rundown:
The Done Deal Message Board is a good thing to read through during the writing your first few screenplays. By looking at the comments there you'll get a solid sense of what rules are pretty important and what rules aren't. Also, when you finally send out some queries if you get a few responses from places you've neither heard of nor can find on the internet, you can post a question and get the lowdown.
John August does a little question and answer thing for IMDB's Ask a Filmmaker feature. He gives even more free advice at his website so check it out.
The Artful Writer has some general screenwriting advice and a bunch of writer's guild related advice on the legal angles.
Finally, John Rogers's Kung Fu Monkey is another screenwriting blog, but this guy also plays Dungeons and Dragons and the like (he even includes a link to the premiere d20 gaming site on the net) so some extra kudoes go that way.