Before you can afford to enter a rights management firm etc. there are a few easy steps you can do:
– If you are a composer print your composition
– If you just play your own music, make a recording of that music
Put that music on a data device that holds up a long time. Now there are long time storage USB sticks or high quality CDs that you can burn.
Put this in an envelope addressed to yourself and send it to yourself via a postal service. (So that a timestamp is on the envelope).
DO NOT OPEN THIS ENVELOPE WHEN YOU GET IT BACK! (best is if the envelope has a seal, like those FedEx envelopes)
Put this envelope in your archive.
This way you can prove to court if ever need be, that you are the originator of that work at the time of posting this envelope.
All other stuff like signing up with a rights management company etc. can come later and they will usually also accept those envelopes that then get opened by a lawyer and stored in their archive.
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As for getting it out there: I would start on soundcloud before iTunes. Get a few people to follow you on soundcloud and facebook and post on other social media with links to your soundcloud. If this takes off then go for iTunes. There is a lengthy article how to get on iTunes on the apple support page.