Hello solotime, welcome!

The stamps grading goes by condition and eye-appeal, mostly somewhat like coins' grading.
Centering is a big one to look for, all the paper intact, no rips, tears, thins or creases, back or front, a nice cancellation if applicable (light, readable), and nice colour or not faded is also good.
After that the rarities and varieties that some stamps have will need to be looked for in special catalogues or the Scott US specialized catalogue, generally available at your locoal library, in the reference section or out on the shelves for loan.
Pre`1940's stamps are generally worth more, if in very fine shape and older stamps than that have their own special grading system. Look for the front pages of a Scott catalogue to read more about grading stamps.
Or, post up some picture here , scanned or with camera and people here could help you with the general valuing or desirability.