To give you a better answer, we need to know a little more. Did it go flat or is it just old and needs to be replaced? We suggest you take it to a tire dealer or your authorized dealership to have a technician look at the tread of the "good" tire and see if it needs to be replaced as well. It would be best for a tech to look at all of the tires. You should also make sure you get them rotated when you get new tires.
If you send us more information, we can give you more details! Good luck!