These sources will help you in all your writing endeavors.
Scripttoolbox is quite frankly the most complete script formatting guide online. Any questions you have about dialogue, transitions, the nuts-and-bolts of writing, you'll find your answers here.
Done Deal Pro is a great site that collects up-to-the-minute Hollywood news and also has a free forum covering a wide range of script topics. Check out the thread about ScriptGal.
For writers in Los Angeles, I highly recommend The Writers Junction - it's a shared workspace, and great resource.
There are hundreds of contests out there, but these two are the only ones I recommend.
Trackingb - winners have a great track record of finding representation and getting projects sold
The Hollywood Creative Directory is the known as the phone book of Hollywood for good reason - the offer annual publications of Creative Executive contact information, as well as a Representative Directory - which lists agents, managers and attorneys.
IMDB - The Internet Movie Database is exactly what its title implies - a database of every movie, television show, actor, writer, crew member, etc. An invaluable resource.
Register your screenplay and get answers about agents, managers, etc.