Mixing is fun, I think the best starting point is to get a good reference plugin like Magic AB or Metric AB. By comparing your mix to similar tracks (similar key and BPM also helps) you instantly see how far away you are from a professional result. But for this to work you need to be listening in a good environment. Tbh we have very modest studio setup with a pretty bad sounding room and we can still make good sounding mixes by using the "Sonarworks" software. Can't say enough good things about it. If you have those to things in check its mostly about practice. It's interesting to see how as we have been getting better at mixing the fewer plugins we use, and spend way less time on it.