It is so complicated that it cannot be explained simply. Anyway, I would say you can safely use the plain form when you are in an obviously superior position to the person you are speaking to, for instance when you are a teacher speaking to your students, a boss speaking to your subordinates, or a mother speaking to your children. Otherwise, it would be safe to always use the polite form. You could use the plain form to your friends, and actually this is the way in which native speakers usually do, but I cannot assure that in that way you will not be regarded as too friendly. So I recommend using the polite form until your friends advises you to use the plain form to them.