No worries, the servo horn is the the little plastic (or alloy) thing that screws to your steering servo. Your steering rack should then be connected to it somehow. The drag link (everyone has a different name) is the bit that connects your left and right wheels turnbuckles so they turn together when you turn the transmitter.
Or the servo it's self might be shot. I had a reasonably good hitec servo that didn't centre it's self properly after a crash
You can check for this on your pit bench. Get your servo into neutral position (unloaded) and turn left and right, then see where it ends up after a few tests it should return to the same spot every time.
But more often than not it's just the settings left to right like i mentioned above. Do you have a set up station?