Begin by "thinking out loud" or brainstorming about possible experiences you have had that might work as topics for the writing assignment. If the topic is "literacy," you might ask questions such as, "What sorts of experiences have you had with language that taught you something about the nature of language, or how language works?" Write these possible topics on the board (or on an overhead). A brainstorming session might produce topics such as regional dialects you've heard or spoken; becoming part of a new community, i.e. marrying into a family of sports fanatics; traveling to a foreign country; learning how to communicate with your pet; learning the jargon of the composition community, etc.
Then, select two or three of the topic ideas. Generate a list on the board or overhead sheet of all the different subtopics you can think of that are connected to those ideas. List out some of the common expressions of the dialects you know, for example. Or do a comparison between dialects. What was it that bothered you about being around all the sports talk?
After you've generated your lists, ask your students to help you find possible connections between the subtopics. Perhaps you notice that the two dialects are really similar in some way, i.e. that they both use jargon that seems to reflect the ethnicity of the original settlers of the area. Or maybe, the speed of speech in each region seems to match the pace of life there. This part of the exercise should be messy, because you are still brainstorming, recording random thoughts.
Then, ask your students which of the topics you've listed seems like it will provide enough material for a three-page paper. Cross out those topics they think won't work.
Then, have students generate their own lists using this same process. After they have eliminated topics that won't give them enough material, have them pick two of the best topics left and start freewriting. As they freewrite, one of the topics will usually emerge as more interesting to them and more relevant to the idea of literacy.