One aid in learning to write novels is to read those already published to see how the professionals do it. We learn from those who know the tricks of the trade. It makes sense that you should read screenplays if you want to write screenplays. Just as a novelist can read a book in two different ways, for pleasure and for analysis, a screenwriter can enjoy a movie for its entertainment value and then study the script to better understand how the movie was created.
The Internet offers many sites to help the aspiring screenwriter. For example, Drew's Scripts-O-Rama (http://www.script-o-rama.com/table.shtml) offers hundreds of downloadable free scripts. The site even labels the different types for you, for example: First Draft, Shooting Draft, Revised Draft, Production Draft, and Director's Revision.
Drew's scripts and other online sites like it have many different sources for their scripts. Sometimes a dedicated fan lovingly transcribes a movie. Sometimes a cast member scans a draft into a computer and posts it to the Internet. Typos and misspelling abound in many scripts, but you'll see that in original works as often as you will in fan transcriptions. Still, it can be very educational to read several versions of the same script, whatever their quality, to see how the film developed during production.
Movies are collaborative works and major changes often take place on the spot during filming. Published scripts will rarely be the same as the movie unless they were written for publication after the final-cut version of the movie was released.
A site called The Source (http://www.thesource.com.au/scripts.htm) has a large online spec script library plus other odds and ends. Spec scripts are short synopsis of scripts with details about how to contact the author if you're interested in seeing more. For example:
"When Life Gives You Lemons
by William Wierzbinski, Jr.
The story of Tommy, Jennifer, and Ariella, three high school friends thoroughly wronged in life by others, and what they go through to get a little justice for themselves."
If you need tools for your screenwriting, The Writers Store (http:\\www.writerscomputer.com/cgi-bin/SoftCart.exe/store/writing_production_software.htm?E+writer) sells screenwriting books, magazines, and software. Some of the software will merely format your text into proper screenplay layout, but, if you're willing to pay for it, you can also buy formatting tools like an index card system that lets you rearrange scenes on fully editable on-screen index cards. There's also software that teaches you how to write as you develop your script, or at least that's what the ads say. As with all software, make sure that your computer is compatible with the software's requirements before you buy it.
Screenwriting Tips for Playwrights (http://www.vcu.edu/artweb/playwriting/screentips.html) offers helpful hints on what works on screen versus writing for the stage, plus lots of good generic tips.
Once you have something to sell, Hollywood Creative Directory Online ( http://www.hcdonline.com/default.asp) bills itself as "the premiere source for contact information for the film, television and new media industries" In other words, ways to get the phone numbers, addresses and email addresses of the industries' top players online.
A site called Moviebytes ( http://www.moviebytes.com/) deals with industry news such as who's sold what to whom and where to submit screenplays for various competitions.
The Writer's Guild of America ( http://www.wga.org/) offers many services, including a registration service (or Intellectual Property Registry). Be sure to check out their Research Links, sites that have been chosen specifically to help writers research their work. This is one of the best reference collections for writers on the Internet. Note that several links are repeated under different categories such that, for example, a forensic medical site might be listed in both Science and Law & Crime.
Saving possibly the best for last, The Online Communicator: Writing for Film ( http://www.communicator.com/writfilm.html) may be the only screenwriting site you ever need visit. It's a gold mine of useful information and Internet links, including "The Craft of Screenwriting" and "Screenwriters and Playwrights Site." There's also a link to "SCREENTALK" magazine, whose web site includes a film script archive that must be seen to be believed (especially for Hitchcock fans).
There are online screenwriting schools, too. I hesitate to recommend any one in particular because I simply don't know which ones are good and which ones are not. As with almost any subject, if you stick with better-known names you'll probably do well. For example, scr(i)pt magazine offers reasonably priced online classes that are probably safe for the beginner to try out. You can find other sites by searching for subjects like "screenwriting class" or "screenwriting". There are also several free online lists of tips and suggestions that call themselves "classes".
Take some time to explore screenwriting on the Internet and you're bound to find something that interests you.