Setting basic level and pan are usually the first things to do in the process of mixing. Choose a sound/channel, e.g. kick drum, to act as your main level reference, and balance all the other instruments tracks against it. So establish the initial gains and then refine with dynamics processing and stuff. That’s what I usually do.

But – here’s a neat trick to help you get the balance right: use pink noise as level reference and balance each sound/channel to it.

Generate or play pink noise at the stereo bus. Calibrate the noise to a sensible reference level that allow ample headroom on your master bus when mixing. Use an averaging meter, a RMS-type meter, to establish the level of the noise.

Start with soloing the first instrument and play it alongside the pink noise, and balance it directly against the noise by ear. That is, try to find the level at which the instrument is just audible above the noise, but not hidden. Now mute that instrument and solo the next one. Repeat. Kill the noise and voilà!

Mixing this way won’t make it perfect, but accurate enough for a start and then some.

Another (general) tip is to listen to and learn by mixers that are much better than you, and that you admire.

Note: Pink noise is a random signal, filtered to have equal energy per octave.