Here you can find all kinds of scopes that are suitable for airguns. Because of the (double) recoil most airguns have, it is necessary your scope can handle this.
Scope are available in different sizes en zoom functions. What u mostly see is for example: Hawke Vantage 3-9x40 Mil Dot
3-9 designates that this scope has a variable magnification from 3 to 9x.
40 designates the diameter in milimeters of the largest lens (objective lens). The larger the lens, the more light input, and thus the more brighter the image.

Further more you can find mounts to be able to mount the scope on your airgun.
Airguns mostly use 11mm Dovetail rail system, but this could be different on some airguns.