In the end, the relative levels of the different tracks (instruments) in a mix is a matter of taste and it's an artistic decision. However, as you are just getting started, there is one basic skill that, if you master it, will help you acheive whatever relative levels you want to have in a mix.
The hardest mix to make is one where all the instruments have essentially the same loudness. You might call such a mix "completely balanced". Normally at least one instrument should be emphasized (almost always vocals), but it's much easier to control which instrument has the emphasis when you know how to balance all the other instruments.
So, as you start mixing, work towards complete balance. There's a trick to telling how balanced a mix is. Get your mix set up and start playback. Then, slowly turn the volume down lower and lower. Any instruments that are too low in the mix will drop out first - you won't be able to hear them but you'll still be able to hear others. If all the instruments drop out about the same time, or stay audible no matter how quiet you make it (until it's off, of course), then you probably have a good balance in your mix.
You can also use that trick for balancing the frequency range of any one instrument. If all you hear is the low end of a bass guitar as you lower the volume, then perhaps there needs to be more mids and highs (assuming you want that sound to be balanced). Note that even in a well balanced mix, it's often true that the drums are still barely audible when everything else has dropped out, don't worry about that. Do worry about whether the snare and the kick are still audible all the way down. They normally should be balanced against each other.
I'm not saying all the best mixes are balanced, but I am saying all the best mixers know how to acheive balance in a mix, and they use that to set the baseline for the mix, so that they can artistically make some things louder and some thing softer than the "main mix" (the balanced part).
