Find a mastering engineer. Mastering is not a process that can be done by an amateur. Even if you know how to mix, getting a good master track takes a lot of technical knowledge and hands-on practice. Find mixes of smaller bands that you like and ask where they got their tracks mastered. Otherwise, look for reputable mastering engineers online, and read the reviews of their work before submitting your tracks.
Play each track individually to make sure that it doesn't clip. Clipping is when the track gets too loud and makes a slight buzzing sound. When you play the track back, clipping is indicated by a red light. If the green bars move up into the red area, then you need to reduce the volume on that track level. Continue to reduce the volume slightly until it no longer clips.
Remove all effects on the master track (if applicable). Some recording programs allow you to put an effect on the entire song, such as reverb or compression. These effects are applied to every track. Remove these effects before continuing on to the mastering process.
Make sure there is no clipping on the master track (the track's overall volume). If you find that it's clipping, turn it down. A good rule of thumb is to decrease the volume to five decibels lower than the level at which it clips. Don't worry if your track sounds quiet; mastering will make it much louder.
Listen to your mix several dozen times at different periods throughout different days before sending it in for mastering. It's important that you're happy with the mix as it sounds. Mastering will polish up your track, but it won't change volume levels on vocals or add a trigger to your kick drum. These are things that need to be done in the mixing stage.
Bounce your track down to the same format that you recorded in. Bouncing the track down will cause all of your tracks to be combined into one single track. This is usually done by highlighting all of the audio files, and then clicking "Audio," followed by "Bounce" or "Bounce to Disk." Set your source to either AIFF or WAV, as these are the highest quality. Set your bit depth and sample rate to the levels that you originally recorded at. Do not try to increase the bit depth or sample rate. Your track should now be ready for mastering.