Hey, there. In my experience, mixing is much more of an art than a science. Here are some rules of thumb that I use as a starting point (many of which probably seem obvious):
1) The focus of the song --- the instrument or voice carrying the melody --- should be in the front of the mix (i.e., louder).
2) I start out mixing everything in mono (no panning). Panning obscures competing frequencies that become obvious in mono.
3) Minimize competing frequencies so each track has its own space. A bass drum and a bass guitar have several overlapping frequencies and can muddy your composition. If I want the kick drum to thump, I'll reduce the lower thumping frequencies on the bass guitar track. If I want the kick drum to pop, I'll reduce the lower thumping frequencies on the kick drum and let the bass guitar carry the lower frequencies. The same concept applies to stuff like acoustic guitar v. electric guitar, etc.
4) After minimizing competing frequencies, give each track its own space by panning. I tend to put kick drum, snare, bass guitar, and the focus (vocal/melody instrument, etc.) down the middle. I'll pan most other tracks (e.q., piano left, guitar right, harmony vox 1 left, harmony vox 2 right, tom 1 left, tom 2 center, tom 3 right, etc.).
Your question asked which should be louder and which should be lower. In my experience, if you follow the steps above, each track will have its own space and can be heard without drastic differences in volume.
I have attached a video that does an excellent job of explaining the mastering process.
http://therecordingrevolution.com/2012/06/25/mastering-tips-from-engineer-and-educator-craig-anderton/
