I agree that the skeleton question above could get you going in the right direction, although I would add that coming up with what a story is about (subject) is just the first step. The next step is to decide what message the author is trying to communicate ABOUT the subject of the story, poem, novel, etc. For example, All Quiet on the Western Front is a novel about war. One theme the writer communicates is that war destroys lives. Another is that war can dehumanize people. A theme statement should include a verb; it is the writer's message or truth about the subject that can be applied to life. Determine the subject, and then ask yourself this question: What message is the writer trying to tell me about this subject?
A theme is any main idea that runs throughout the story. You can figure out the themes just by filling in the following blanks:
____________________________(name of story/play/etc) is a story about ________________________________(theme).
Romeo and Juliet is about many things, but young love, rebellion, deception, love, marriage, violence are just a few. Can you see how these things may be themes? Romeo is always in love, but when he finds Juliet, he is convinced it is the real thing.
The Capulets and Montagues have been fighting for so long, even their servants get involved. Violence abounds so much that the Prince has declared anyone else who does it will be put to death (more violence, mind you).
The main thing about themes is that the idea runs under the surface all throughout the piece of literature. You should be able to think of examples of it from the beginning, middle, and end of the work.
Check eNotes study guide for a lengthy discussion of various themes throughout the play.