There are several examples that can be found on the Internet for DNS configurations.
http://www.irandata.com/linux/dnsserver.htm is one example to get you started. You should first set up the DNS server and test it by pointing a different computer to this as the DNS server and make sure that it works as you want it to. Slackware should have a default /etc/named.conf and a basic caching example in /var/named. You'll have to add a file for your domain.
Then you need to go to netwisp, (presumung they are hosting your DNS) or whoever you registered the domain with, and change the primary DNS server to your IP address. You should also ask your current DNS hoster if they will be a secondary DNS to you, in case your server goes down.