Writing screenplays or anything else that falls into that general category is a very good exercise. It forces you to focus on small details and good dialogue.
Learning to format a screenplay is tricky. There are several good program options. See a list here. I picked the Celtx program because it was free, looked easy to learn, and didn't take much to download.
I enjoy finding screenplays online and reading them along with the film. This has been the single best learning experience. I also have a slew of books that seem helpful although I haven't got into them near as much as I'd like. I should work up some reviews soon.
There are several social sites for screenplay writers that look helpful. I don't have any suggestions because I haven't joined any. I don't want that sort of distraction when finding time to write is so precious, but I think these sites can be very helpful and encouraging.
If you are a screenplay beginner I would look into Script Frenzy to get a good start. :-)
Let me know if you have any advice or experience in this area.
Thanks for reading,