To get started, first go to Playwright 101 and spend some time looking around.
You will first need to decide what kind of play it will be, go here to do that. Next, consider your theater space and how you want to set up your stage/theater by going here. When you're ready to start writing go here and learn about the exposition, the complication and the resolution.
Because you already have a text to work with, your main job is to decide on what stays and goes. Use what you've learned from the resources above to create your play manuscript.
Use the Table of Contents links 5-15 to understand what your written play, dialogue and stage directions should look like. Don't forget about your Title Page and Cast Page.
Don't be afraid to take advantage of the glossary page. It is there to answer your questions (and help you sound like you've been writing plays for years...) Use it.
Next, in developing your characters, take some time to read up on what your characters will be wearing. Do you want to stay true to the text or design your own costumes? What props will you use? What music (if any) do you want to use? Go here to look at some of the work other people have done on the novel. Listen to the music and check out some of the links. It may inspire your choices on costume design, music, props.
Finally, rehearse, rehearse, rehearse...