Here is how to set the c1000a into bridge mode to function as only a modem:
http://internethelp.centurylink.com/internethelp/modem-c1000a-adv-bridging-q.html
Before changing these settings see if you have a static IP or dynamic IP from century link because if it is static you will need to copy the ip, subnet and gateway settings.
You will need to go into nighthawk and set it up for PPPoE connection in wan settings and you will also need the PPPoE username and password from century link.
While you are in these settings you should go ahead and set DNS to 8.8.8.8 and 8.8.4.4. This is Google's free DNS that is usually faster and more accurate then relying on your ISP.