Just write. This is perfect for students who already keep a journal and enjoy writing about anything and everything, but it can also work if you feel lost at the beginning of an assignment. Beginning with a fresh sheet of paper, write anything that comes to mind. This can be a description of someone you know, random words or images that come to mind, or even a detailed account of what you ate for lunch today. Whatever it is, don't think - just write.
Once you've written for at least five to ten minutes, read over what's in front of you. If you see a word, idea or image that pops out, circle it. Many students find that this is the fastest and easiest way to come up with a creative writing topic.