Contrary to how it sounds, secondary sources are the sources that you should use first. They'll help you track down relevant statutes and cases, and can save you huge amounts of time, as well as ensure that you don't miss relevant authority.
There are five major types of secondary sources:
You may not need to consult all of these every time, but it's important to check each of them to see what they can offer. Many times, you'll find that a secondary source has done all the work of identifying and organizing the authority you need. They're the experts; use them!