Copyright and Copyrighting!

[ChezUK]

I've created a mix from various acapellas and instrumentals and some royalty-free samples.

What is the legal position of using copyrighted instrumentals and acapellas (I assume they're copyrighted as they are instrumentals and acapellas of well known songs?)?

I assume you would need permission from the copyright owner to release a track using them?

Assuming that is correct and assuming permission was given (unlikely, I know, but humour me!), what would I have to do to ensure that my new track(s) weren't stolen by someone else?

Is there an official route to take in order to get my track(s) copyrighted?

Many Thanks
Richard

EDIT - In case it makes a difference, I live in the UK