The domain is mine and the ns is already pointing to x10. I just want the default domain on my account is ralphweb.net.you have to purchase ralphweb.net from a registrar then change the Nameservers to those of X10 .. After that, you can link the new domain with your account .. but take into consideration the limitation on your account still apply.
For free hosting accounts it is required that the main domain is one of the subdomains we provide.
To use a personal domain you would need to set it as addon (as suggested above) or as a parked/alias domain. The later will make the domain show the same content as the main domain does, an addon domain would be used primarily if you want to make a second, separate site (site would in this case be served using a subdirectory of public_html as the domains root).