Two Apple IDs competing on my Mac

HI

After my previous mac died, i got a new computer MAC at the SoHO store in NY. Since i did not remember the password to my Apple ID on my old computer which was registered in Belgium, the sales person told me to register a new AppleID on this computer. I did so. However - ever since, I am getting pop ups of apple ID prompting me to sing-in with the old AppleID . I'm worried that I have been backing up to the wrong account. Please help me stop those prompts...