I don't know what kind of English you're taking this year but I am also in 12th grade and am taking an Advanced Placement English class. Just today our teacher actually told the class that a reoccurring problem she sees within our essays is when
the prompt asks us to write about a theme found within a book or play. She said that a theme can't, or typically won't be, a one worded answer. She gave us the example of friendship and told us that friendship alone can't be the theme. What about friendship is the theme. She told us to take that one word that we think of and elaborate on what the book/play's reoccurring message of it is. So in Hamlet, ask yourself, "what about poison does the message of Hamlet give?"
Hope that helps you steer a little in the right direction. :)