Learn the lingo. As with most fields that require special training or knowledge, digital mixing has its own language. This includes many acronyms and abbreviations to describe the processes involved in mixing music. Read trade magazines such as Computer Music in order to learn the technical jargon. Look up unfamiliar terms if the context of what you're reading doesn't explain them.
Study input-output theory. In audio recording, input-output theory is essentially the basis for all playback and recording. Every audio source (out) must be routed into a playback source (in). For example: a CD player (out) must be plugged into a receiver (in) that has speakers in order for you to hear the audio.
Download Audacity. This free digital audio workstation (DAW) allows you to record live audio, chop, edit and splice MP3 or WAV files, and has a host of other features. Audacity isn't as advanced as Adobe Audition, ProTools or other software, but you can get the basic training you need by experimenting with Audacity.
Purchase a mixing board. It's possible to use Audacity to edit pre-recorded audio that you import. However, to get a real feel for mixing digital music, you will want to record live audio of your own. To do this, you need a mixing board. Get one with at least one line input channel and one mic input channel.
Obtain an amplifier if the speakers you're using are not powered. If you're using a speaker setup such as the one that came with your computer, simply route the speaker out of your sound card into the line in of your mixing board, and route your mixing board main output to your powered speaker box. Using non-powered speakers, your computer will route to the mixing board, the board to the amplifier, and the amp to the speakers.
Record and edit different sounds and beats. Now that your setup is complete, you're free to experiment with mixing and editing digital music.
Apply for radio internships. An internship in broadcasting will further train you in the basics of sound boards and digital mixing.
Attend vocational school. Full Sail University is one of the best for associate and master's degrees in audio production.