Reading a tyre size
Take 90/90-21 as an example. The first number is the width in millimetres, the second is the sidewall height as a percentage of the width, and the last is the rim diameter in inches. An R before the rim size, as in 150/70R18, means a radial tyre.
Size is only half of it. Match or exceed the load index and speed rating printed on your current tyres, and know whether your rims are tubeless or tube-type: a tubeless tyre on a tube-type rim still needs a tube.