*Sigh* your ISP wont be able to open your ports for you.
Log onto your router, 99% of routers are accessed through an internet browser using the address 192.168.1.1. You'll need your router username and password, again 99% of these default to admin (or administrator) and password.
Once logged into your router find a section called Port Forwarding. Before you navigate to this screen ensure you know your Xbox's IP on your network (something like 192.168.1.X) and your Xbox's MAC address. Once you've noted these down go to the port forwarding screen. In the UDP fields, both to and from, enter the number 3074 (This is the main port for Xbox live) then enter your Xbox's IP in the appropriate IP field and MAC address if nessesary. Save your settings and restart your router. You now have an open NAT.