For selecting the right propeller you first need to know what is currently your type. The type is determined by the diameter and pitch. This is mostly indicated on the outside of the propeller hub.
It is usually written like this: 13 1/4 x 17 or 13.25 x 17
The first number is the diameter (13.25) and the second number is the pitch (17).
Note for Yamaha outboards: sometimes a letter is added at the end (like the G in this picture), this is a brand indication, but is not related to the size.
Note for Mercury/Mercruiser engines: the number on your prop may look like: 48-832832A45 21P. This last number (21P) indicates a 21 pitch. You can now choose a propeller in our shop with the same pitch. The most common 48-numbers are also mentioned at the product page.