There are so many domain name registrars and resellers. Sirisoft is a domain name reseller.
Go to
http://domains.sirisoft.com.auSearch for a domain name you want, and if it is available, just follow the instructions to register it.
When you register a domain name, you have to select the length of time to register it for i.e. 1 year, 2 years, etc.. Once the period is up, the name has to be renewed, otherwise, it is unregistered.
-Seng