Do you recognise anything about the email address, have you had a yahoo.com mail address starting with 'r' ? If it's your account then you could see if you can get access to it via http://iforgot.apple.com
Or did you buy the phone used, in which case could it be the previous owner's account - if it is then you will need to contact him/her to remove it from their account : http://support.apple.com/kb/TS4515