I have some experience writing stories, but would like to beginning screenwriting. Are there any online guides to show how to get started and transition into writing for the stage, where ideas need to be presented through dialogue and props, rather than narration.
I would say start by reading some scripts. You need to see the style they have, and the best thing you can do (other than start writing yourself) is to read the work of successful writers. You can learn how to write good scenes, and, in the case of screenplays, you can watch the movie as well to see how it all looks once done.
The Daily Script is a great website to read screenplays for both TV shows and movies. I'm sure there are similar sites for plays.
Good luck and have fun!
The following applies more to playwriting than screenwriting.
Stuart Spencer's The Playwright's Guidebook is the best book I've seen on the subject. It's practical and accessible but not dumbed-down, and built from his own experiences in the theater world.
On top of reading books and studying plays, I would say start going to the theater regularly. Plays are meant to be experienced, not read. To go even further, donate your time to a local small theater group. This might mean setting up and tearing down sets, lighting, even acting, but that's part of the deal. Not only will you see how theater "works", you'll also make contacts with the very people who produce the work of young playwrights.
A book I really like (and full of useful tips) is "How Not to Write a Screenplay: 101 Common Mistakes Most Screenwriters Make" by Denny Martin Flinn. He goes over examples of bad screenplay writing compared to bits from screenplays of some big films like Braveheart and Bladerunner.
I was on the same quest, and thought this book was gold:
It really throws you in there, and teaches you the typing structure in a brilliant way. Not the most comprehensive book on the subject, but the best I have read for people who are just starting out.
I would also highly recommend a good writing tool, like Scrivener. Screenplays rely heavily on formatting, and with the right tool, you won't spend much time hitting the tab and shift keys.