As an example, my tire size is 235/45/18. This means the tires are 235mm wide, have a 105.75mm total side wall height (235mm x 45%), and they fit over an 18" Ø wheel.
You need all these numbers, because a 305mm tire width would not fit my wheels, nor would a 195mm tire width. The second number (45 in my example) significantly influences ride comfort and tire noise, and can potentially cause your tires to rub on the fender if spec'd incorrectly.
I would recommend sticking with whatever the manufacturer recommends - you can find this on a sticker on the driver's door, in the owners manual, or on any number of websites. If it's a relatively modern car, there should be plenty of options from reputable manufacturers (ie Bridgestone, Toyo, Pirelli, Falken, Michelen, Goodyear, Yokohama).