Did you Google for "Linefollowing robot nxt-g" ? I found a couple in the first 5 hits.
You should check out the nxtprograms.com
website, it has a LOT of very cool robots to get you started, including a line follower
| My Blog: I'd Rather Be Building Robots (http://botbench.com)
| RobotC 3rd Party Driver Suite: (http://rdpartyrobotcdr.sourceforge.net)
| Some people, when confronted with a problem, think, "I know, I'll use threads,"
| and then two they hav erpoblesms. (@nedbat)