When first learning about a topic, use background sources like encyclopedias, handbooks, almanacs, etc. to get a clearer idea of what you're looking for.
Look to your assignment to see what types of sources your professor is asking you to use. If you're already comfortable with a topic, you may start looking for scholarly articles right away.
You can begin your research in any of these places. Remember different types of sources contain different kinds of information.
Credo is an academic alternative to Wikipedia. It has topic pages for many biology and science topics that can help you get a better idea of how to form keyword and phrase searches in article databases.