First try to understand why the experts of the topic are interested in it (Blogs, papers, books, personal contact, whatever). Look for books on the subject, test-read each one, settle for at least two that are written on a level that seems accessible to you (if there are any, i.e. if the topic is not too hot). Write down every formula, mark every sentence that seems important. (Note that different kinds of peaople need different kinds of techniques, some need graphics, some need to speak out loud, I need to write. Works for me in everyday life as well, as soon I have writen something down I have memorized it, well, at least for a while). Try to explain everything you read to yourself. Repeat this until you are sure you could do a talk without any notes, anticipating all sensible questions.