This is just a basic description to help people create more clear, and better sounding vocal submissions that they would like evaluated.
Mic Level Input: You want to have the strongest signal you can coming in to the recorder. I usually set my vocal levels to PEAK at around -3db. That is the highest level that I want to see during my loudest vocal volumes. Even if the lowest volumes only peak at -15db you're still good.
Mic Level Output: This is your main output. You want to match the volume of your input (mic) to be the same as the volume of your output. (the outputs will be in stereo so you want to match both output meters to match the mic input). Hit record and go!
Mixing your vocals to backing tracks: I always set my music low. Usually around -15db, and let the music peak at around -20 db. I set the vocals over it using the same levels that I used during the recording process. The overall mix should not exceed -0db at its highest levels. I always set them a touch lower to ensure there is no clipping.
This is a very basic description, and is a basic principal for any software, or hardware you may have. If you want to get into it deeper just ask.