Click on the menu on Project and then Import audio, then choose the music file you want to import. Audacity allows you to import  WAV, AIFF, MP3, OGG VORBIS, AU audio files.
It opens now a new audio track (or two, if the imported file is stereo).
You can mismatch the start of the two tracks by dragging a track or the other with the Time shift tool.
There are e two methods to adjust the sound volume rate between the two tracks:
- by adjusting the amplification of each track with the gain cursor situated in the left side of the track.
- by using the tool Envelope that allows to vary the volume time of each track, allowing for instance  to lower the volume of the soundtrack in a speaking moment.

Example of mixing voice (a mono track) with music (two stereo tracks) by using the Envelope tool to lower the music volume in a specific moment.

You can now go on to the section Improving the sound.