Start by listing questions related to your topic. What do you need to know in order to write knowledgeably on this topic?
These questions might include basic statistics (How many people have been diagnosed with Type 1 diabetes?), context questions (Does Type 2 diabetes affect a certain portion of the population--by age, race, sex, or ethnicity--more significantly than other populations?), etc., etc.
You can then turn to reference sources--dictionaries, encyclopedias, and books written for a general audience--to start to answer these questions.