A GOOD WAY to use compression,
is incrementally.
meaning, a little here, a little there..
there's two ways of looking at compression.
1, 'going in', meaning, applying compression of the source before it goes into your daw.
this means outboard compression.
2, 'after the fact', which means that something that has already been converted from an analog signal to a wav file, gets compression.
after the fact.
compression can be used as signal level control,
or used for 'color' of the sound itself.
it can be used to gently shape a signal,
or beat the ever loving daylight out of it.
and everything in between.
no one can teach you how to get good at telling the difference, or exactly how to use it for every kind of signal you will ever grab,
but there are tons of 'guides' to help you along.
when you talk about overall song level,
you are getting into another area of mixing, and that has to do with how you sum your tracks at your mixer,
and whether you process them to get more gain out of the overall level or not.
as a beginner, i just would not even worry about that, until you get the basics down first,
and with theses kinds of questions, it's obvious you are just getting started.
MASTERING......
the next question.