If you are replacing your current car tyres the size and specification is marked on the tyre sidewall.
There have been cases on some second hand cars where at some stage in the car's life the wrong size tyres have been fitted.
By checking your car's handbook you can find the original tyre size and specification you should be running on your car.
Using tyres not originally specified for your car can be dangerous and seriously affect the handling.
Mixing different types of tyres on the same axle can be both dangerous and illegal.
Obviously the spare tyre supplied with the car is an exception to this rule.

Run-flat tyres
Many cars are now fitted with run-flat tyres.
These tyres enable you to continue driving your car with a puncture for approx 50 miles at a maximum speed of 50mph. This can be achieved as the run-flat tyre supports the vehicle's weight.
The tyre markings indicating the tyre is a run-flat vary by manufacturer
These include DSST, RFT, ROF and RunFL.

It is recommended by Tyresafe that run-flat tyres shouldn't be used on cars without a pressure-monitoring system, a system which notifies you of a low tyre pressure or puncture.

Reading your tyre markings

The sequence of letters and numbers on your tyre sidewall display the size, dimensions and many other features of the tyre.
Sometimes they are not that easy to read but they are always displayed.

Here's what they mean.

185 = the tyre width. This is in millimetres.
60 = the tyre sidewall profile, this is the height of the tyre sidewall based on the percentage of its width.
R = the tyre is a radial
14 = the diameter of the tyre/wheel rim. This is in inches.
T = the tyre's speed rating, it is a legal requirement that this must match or exceed the top speed of your car

Here is a list of all tyre speed ratings.
S 180km/h or 112mph
T 190km/h or 118mph
U 200km/h or 125mph
H 210km/h or 130mph
V 240km/h or 149mph
W(ZR) 270km/h or 168mph
Y(ZR) 300km/h or 186mph
ZR Above 240km/h or 149mph