I Cant Connect To Xbox Live...?

So im trying to connect to xbox live but i keep getting this message "Your NAT type is Strict (or Moderate). People on networks with this NAT type may not be able to join certain games or hear other people while playing online." i kow i need to change it too open but i dunno how....can someone helo me...by the way i have at&t and im using a modem..helo