How do you claim a domain name?
The trick to acquiring the domain name you want is to use a trusted registrar to (a) find out if it is available, and (b) to register it for yourself.
It is important to use a legitimate registrar, such as pairnic.com, so that you don't inadvertently alert scammers that you want that particular name. Scammers have ways of tracking which domain names are desirable and will pounce on ones that are searched. To avoid losing your domain name in that way, stick to the trusted registrar sites to do all your research.
Once you have determined that the domain name you want is available, then you can use that same trusted registrar, or the registrar with whom you already have sites registered to claim it for yourself.