I'll set the ball rolling on this one, but music copyright is not really my area. You don't say where you are living, so I'm basing this advice on UK law, but it is broadly the same throughout the EU. First of all you autonatically have copyright in something you created, so it is not essential to register anything. That said, to protect your intellectual property there are steps you can take to help you prove, at some later stage, that you created something first. From what you've described, there are two separate things here: music and lyrics both carry their own separate copyright. Make sure you put both down on paper (if you can't write the score of the music, put an audio version on a CD) and find a way of 'date-stamping' your creations. The simplest way is to seal copy of the music and lyrics in a letter which you mail to yourself by recorded mail, and keep the unopened letter in a safe place. This is not foolproof, and a better way is to have the documents bearing the words and music notarised and lodged with a lawyer, or use a copyright registration service (such as
http://www.copyrightservice.co.uk but there are others). In the case of the music you created with someone else, this might be viewed as joint authorship if you both contributed something original to the song. In that case, the other person would have copyright as well as you, and you would need to share the royalties on the music (but not the lyrics). The next stage, involving the DJ remixing your song, is known as a derivative work and he needs your permission do do this. However the DJ would then own 100% of the copyright in the new remixed song. A simple letter agreeing that he can use your song would cover that bit, but I suspect that you would want some royalties from him if his derivative work starts making money, and this could be based on either sales, the number of plays or a number of other formulae, or just a one-off payment. You could draw up your own contract, but given how complicated this is, it might be better to get one done by a lawyer with experience of this area. The matter is further complicated by the fact that you are going to sing on this recording. Your performance also has copyright attached to it. So as an example: in the remixed song, you will own 100% of the lyrics copyright, 50% of the original music for which royalties need to be paid, and some of the copyright in the performance, depending on how the music is created, produced, arranged etc.
And as if that wasn't difficult enough to get your head round, you have the international dimension. I have no knowledge of Russian copyright law (
http://en.wikipedia.org/wiki/Russian_copyright) although Russia is a signatory to the Berne Convention, so in theory this affords you fairly good protection. Spain, in addition to also being a signatory to the Berne Convention, is also governed by various EU Directives on Copyright.
I hope that someone else will be able to fill in the gaps in my knowledge on this subject, but I think that you should get some professional legal advice. Depending on where you live, maybe you could contact your own country's Musician's Union or the collecting society for music, for further advice.
