The music industry completely changed over the past decade. More people are switching over to the digital music platform. Even streaming is even more popular now with data getting faster and cheaper.
Nowadays it is fairly simple to sell your music online. The most important factor is "Copyright"
You are not allowed to simply record(cover) any artists song and upload it on iTunes. It does not belong to you. You need to get permission from the artist/composer or copyright organization. You will also have to pay fees(Royalties) to the composer/artist.
You need to copyright your own composed music before you could distribute it online. The simplest way is to contact your country's local copyright organization to assist you. It could also involve membership fees. Another method is the Poor man's copyright procedure.
You can read up on this method here.
Then you need to find a company to distribute your music for you. They distribute and collect royalties on your behalf.
Here's a list of good distribution companies:
Tunecore
CD Baby
Tuneport
Reverbnation
