How to change NAT settings for modem with no router?

My modem is connected directly to my computer with a ethernet cable, and the usual internet cable into the wall. Im not a computer wizz, but ill give all the details I can. The modem itself has the logo name StarSmart on it, it was issued to me by my local internet provider. Im trying to play on xbox live with the modem hooked up to the Xbox 360 directly without a router and it says that the NAT settings are set to strict, if you guys could provide a guide for my feeble mind to set it to open, that would be great! Thanks in advance.