So, start simple.
Don't expect to write one sketch that will do everything from day one.
Break down the problem.
Work out how to connect your H-bridge, and then write simple routines to start, stop and reverse the motors.
Check these out on your robot chassis.
Make sure that you can control the speed too.
Then check that you can make sense of the readings back from the line sensor. You may need to calibrate this for your particular course.
Have a look around the playground.