The easiest way is to look at the sidewall of the old tire to see what size tire you have previously used. There you will find all the important key figures such as tire size, load capacity or maximum speed and other information.
You will also find the relevant information on the registration certificate part 1 (this used to be the vehicle registration document). Here you can determine whether other tires may be suitable. Your tire dealer can also advise you if, for example, you want to convert wide tires.