Someone on Youtube sent me this link about primers:

http://www.scalemodelguide.com/guide/painting/all-about-primers/

I personally don't use primers very often.   Most model paint, Testors, or Tamiya are designed for plastic, so a primer isn't really necessary.  However if you are building a resin model I would put down a primer coat.

This link has a lot of good articles about paint for the beginner.

http://www.scalemodelguide.com/guide/painting/