There's no real "rules" when it comes to levels for things, it's purely subjective. I would say listen to tracks that you like the sound of and work out where the levels of things sit on those - is the snare louder than the kick? is the bass loud? Are the vocals as loud as the guitars? etc.
Once you have that worked out for a couple of tracks, try the levels on your own stuff. If you mix enough, it becomes a matter of taste and intuition as to where things sit in your mixes.