Why can't I drive my RC Truck straight?

Make sure that all the wheels can are spinning when you accelerate; there might be dirt or something else keeping the tires from spinning correctly (since it's only turning right it's safe to assume that it's one of the right tires).

If the tires are fine, then something's faulty with either the controller or the motor. Try using a different controller for your truck and visa-versa to figure out which one it is, and replace the faulty part.