The easiest and quickest way to learn anything is by watching the pros in action. As far as mixing is concerned, we might not be able to be present in the studio with the best mixing engineers while they work, but we all have access to… their final mixes!
During mixing, it is incredibly useful to have a professionally mixed track embedded in your sequencer window and muted but accessible through just one click (or key) for comparison purposes. Of course, you need to pick a track that’s somewhat compatible with the one you are mixing: similar genre, similar instrumentation, similar tempo, similar vibe. Also – of course – pick a mix you really like.
Different records sound entirely different, but that doesn’t mean they are badly mixed. It’s just a question of style. You’ll also need to consider that the comparative track went already through the mastering process, while yours didn’t, so it will sound way louder and more compressed than the one you are working on – to compensate, simply turn its volume down until it matches the volume of your mix.
Quickly A/Bing similar sections of the two songs can be very helpful in getting the mix’s balance right, and sometimes, when you are particularly happy about a mix you created, it’s a good idea to use it as an A/B reference for other songs by the same artist.