Obviously you must know the size of the tire on the car that you need to replace, you can look at the tire that is on the car and read the size right from the tire and then you can call different tire dealers to compare prices. You also could take the car to the dealer and let him look.
If the car does not have the original size tires on it or you don't have the owners manual then you can look this info up. Go to a tire website like Tirerack. They have a tool to look up tire size info based on year, make , model etc...
Look at the sidewall of the tire. The tire will give you all of the information that you need. The first thing is the diameter of the rim. I am guessing it is probably 16 inches, then it will give you the tire size. Something like 225/75. Read the tire.
You can find the tire size either by looking at the tire if they are oem size, check the door tire inflation sticker, or going online. You can look up tire sizes by make and model on most tire stores websites.
On the tires on the vehicle currently will be 3 numbers - these numbers will tell you the sized of tire you need and can tell a tire salesman what are currently on the vehicle. It's written right in the rubber on sidewall of the tire.
I would really go with stock tires for your Ford Focus. Since this car is really not a sports car it would look really nice with stock. Normally these would either be fifteen or sixteen inch rims. You could probably purchase these tires and rims online at many different internet sites.