There are tons and tons of threads on this, as well as tons of online resources. As just a mixer with no knowledge of mastering, when I need to send out a track that won't be mastered but the volume needs to be brought up, I use a free VST limiter and just tweak a mastering preset. It brings the volume up, but not past a certain point. It does the trick for my purposes. Anything more complicated than that and you'll have to ask a more specific question.
