mixing = everything done before the final mixdown
levels of channels, eqs of specific instruments, compression on groups, whatever etc etc.
mastering = everything done on one single track of the master after mixdown
in the meanwhile mastering should not bother you. get a good sounding mix which sounds like a pro track. mastering doesn't change the sound of a track. if it sounds like shit, it will still sound like shit. mastering is some final touches.
my best advice to you - work on low volume, get a good mix and then put a maximizing limiter on the master to get it sounding loud, such as L2, BBE, etc.