You can get your digital album professionally Mastered and make it available on all the popular stores and streaming services. A ballpark budget starts at around $40 per song for Mastering, plus $50 to digitally distribute your album and you should allow yourself a few months to complete these processes.
If you’re trying to get your music out without spending anything, do your best to “master” the tracks yourself, set up your free Paypal and Bandcamp accounts, upload your album to Bandcamp and voilà! – you’ve officially released your music digitally.
You should save your final mixes as high-quality (24 bit is standard) stereo WAV or AIFF files bounced at the same sample rate you’ve been mixing in. These WAV or AIFF files are what you will submit to the Mastering Engineer, what you will get back from the Mastering Engineer, and what you will then upload to digital stores and services (they will not accept MP3’s!).
To sell your music on iTunes or any of the other major services, you will need to go through a Digital Distributor and they will provide you with both a UPC code for your album (this identifies your album and monitors your sales) and ISRC codes for each of your tracks (same idea, but for individual songs).