adding to the advice of the posters before I'd say to stay clear of compressors and filters, most tracks are produced in software and have already the shits compressed out of them. You could apply normalisation and a mild compression after finishing a mix, for overall gains or individually in preparing your tracks before mixing, some tracks are mastered to an extend that leaves a lot to be desired. In a live situation, e.g. club, house party you won;t have the time to apply filters and such, working with presets is a possibility but could make things worse as all tracks are differently mixed down.
Take a track you feel it could be improved and experiment with some settings available in your software. If you're in need of some stuff try
KVR , as with most things it's a lot of learning by doing and personal preference.
Fixing frequencies individually for a track in a mix is very tricky, solution is proper mixing in the first place and track prepping.