The replacement has to be the size the car is built for, with a load index and speed rating at least as high as the maker specifies. All of it is printed on the sidewall of the old tyre, and most cars also have a sticker inside the driver's door or the fuel flap.
Tyres on the same axle should match: the same size and type, ideally the same model, so the car brakes and corners evenly. If the tyre on the other side of the axle is badly worn, it is worth replacing the pair, and we tell you what we find while the car is up.
Four-wheel-drive cars are more sensitive. Some makers ask for all four tyres to be close in tread depth to protect the transmission, so tell us on the call if the car is four-wheel drive.