When driving the car pulls to the right

Okay, so you are saying it pulls to the left. Then once you manage to get it to go straight it starts to pull to the right? The first thing I would do is check that the tire pressures are correct. Is this a stock Mustang with no aftermarket items like different offset wheels or any type of lowering kit? Next, did this just start happening or has it slowly gotten worse?
Is this a manual rack or power steering? If the latter you may have an issue with the seals in the steering rack failing and fluid is bypassing and trying to turn the car. If you just start it up, turn the wheel gently to the left then let go, does the wheel keep trying to turn? If so then it is time for a new or rebuilt rack. An alignment shop would also be a good place to take it, worn or damaged parts can cause pulling as well.