Step 1: Think about your topic! What questions are you trying to answer?
Step 2: List terms that best represent the main concepts of your topic.
If nothing comes to mind, it is time to do some background research!
Start with MedlinePlus, by the National Library of Medicine, for a general introduction to many medical topics, often written in a consumer-friendly language. It includes an online medical dictionary, an illustrated encyclopedia, and drug information.