i have iphone 4s, i want register to my apple id, but i cant (this iphone is registred to another id..)

Not sure what you mean by register... But see here...

http://support.apple.com/kb/HT4627

Or perhaps you need to do this...

Goto

Settings > iTunes and App Store > Apple ID = Sign Out... Then Sign In using the preferred Apple ID.

It should be Noted that anything Downloaded with a Particular Apple ID is tied to that Apple ID and Cannot be Merged or Transferred to a Different Apple ID

You can no longer register any Apple products with Apple, only setup a support profile. So, even though the phone is registered to someone else, it doesn't matter anyway.