Ideally you need a static IP address for your server, one which will never change. Once you have this, you should be able to change the DNS with your domain registrar unless you have changed the nameservers. Go to http://who.is/whois/yourdomain.com/ to view the Registrar and Nameserver information.
The registrar is the company that controls your domain, and as such the company you need to change any details (such as registrant information or nameservers) with.
The nameservers tell you who handles the DNS for your domain, which will likely be the domain registrar as mentioned above.
Go to wherever your domain is setup (again, probably your registrar control panel), and add an A Record for yourdomain.com pointing to your static IP address. Also add one for www.yourdomain.com.
If you do not have a static IP, there are services such as no-ip to provide dynamic DNS, but I can't speak from experience about them.