You need to know who your domain is registered through. If you do not know (Domains are registered in years or multiples and it is easy to forget) You can do a WHOIS search on many websites. DNSStuff.com has a pretty good WHOIS lookup and it is free. When looking at the WHOIS, you may or may not see the underwriting registrar. If you registered your domain directly, you may contact the registrar for your username and password. It is usually just a matter of clicking a lost password link. If your domain is registered through a reseller, you will need to contact the reseller for your login information.