I agree with what was posted above. Also learning about the frequency spectrum such as the lows ( kick and bass), midrange, and highs ( hi hats and snares for example). Mastering is the final stage so I recommend focusing on mixing and that will affect the master. Getting a balance by using volume faders, panning. Also the song arrangement is important, as the more instruments you have playing at the same time, the more busy your mix will be, which means there will be less space in the frequency range to work with. A little music theory will help when it comes to making the beat. If sounds are clashing, try removing that sound or play it in a different octave. There are alot of youtube tutorials on production and mixing, but be careful because alot of them give out wrong information. I say the more music you make, the more you will get a feel for how to do things. I say practice with eq, compression, reverbs and delays. Time based effects such as reverb and delays are usually placed on an aux track as a send. This gives more control of the reverb and delay. You could also place it as a insert directly on the track, which you will adjust the wet/dry knobs. If placed as a send, the wet/dry knob should be 100% wet. Mixing is hands on and the more you practice, the better you will get. Every song is different, so don't let anybody tell you you always boost or cut a certain frequency. Its about listening to what the song is telling you. As you progress, your ears will be trained to hear certain frequencies. If it sounds good, then it is good.