Aah, if you want your robot to walk, not just dance in place, it’s going to need some knees! You can add another set of servo motors to control them, or if you’re happy with the robot just walking forward and turning you can make some passive (not motor-controlled) knees by cutting the legs in half, then taping them back together on one side with something like electrical tape, so that they only bend one way. I think you’re going to need another support, in addition to your trailing wheel to support the robot while it’s moving its legs, at least to start.
That line-following robot project you link to looks like a great beginner project, go for it!
And please feel free to use any of the code I’ve posted for any of your projects. It’s been made totally public by putting it on this forum, and I probably copied large parts of it from someone else’s code to begin with.
Good luck in the contest!
-Adam