IMHO to get good mixing and mastering you must start with good tracks. Your so-so track could mess up your mixing and mastering times as you try to correct the problem at the wrong places. Correct the problem at the source, i.e. the individual track.
Also subscribe to The Recording Revolution
https://www.recordingrevolution.com/Graham offers great free advice on everything recording from effects, polishing tracks and mixes, etc. This is one free source that is a must have IMHO.