When you look at the side wall of your tire you will see a number that looks something like 245 45 18

The 245 is how wide your tire is; 45 is how high the sidewall of the tire is and 18 is the size of the rim.

You will sometimes have different size tires on the front of your car from the rear of your car and they will sometimes be directional.  I would suggest taking your car to a professional when you need to change or rotate tires.  You also want to try and stick with the same brand and tread pattern of your tire if possible.