ok well YOU need someone to register the domain through, set up the domain (and tell them it is already registered and you own it) - then it depends on what the domain name is for what happens next.

For .co.uk you need to get the original registrar to change the IPS tag on the domain to the new registrars value. For .com I think it has to send an email to the admin email address on the domains DNS settings.

Just register the domain with your prefferred company and follow their advice / help.