[Ash.]

Assuming that your DNS is on the same server as the cPanel administration tool (and that cPanel is managing it), you can do it through cPanel. It's been a few years since I've used cPanel, however if I remember rightly there is a feature called 'Simple DNS Zone Editor' (or look for something similar, I'm probably wrong). From there you will be able to add an 'A Record', which is the record required to map IP addresses to servers. The 'host' or 'IP' or whatever the 'content' field is, will be your SAMP server's IP address.

This may be of some use, though I haven't read it fully as I'm on my mobile device at the moment; http://docs.cpanel.net/twiki/bin/vie...impleDnsEditor

EDIT: nickdodd25's post is the correct way to do it. However, please be aware that you may also have to wait for these domain changes to propagate through DNS server's, which could take up to 72 hours. Any DNS changes have to be propagated!