Cambridge Dictionary gave the following rules that will help us learn how to use “beside” or “besides”
(a). Beside is a preposition. It means ‘at the side of’ or ‘next to’. It is rather formal:
Ex, He would like to take a photograph of us. Would you come and sit beside me?
(b.) Besides is a preposition or a linking adverb. It means ‘in addition to’ or ‘also’:
Ex, What other types of music do you like besides classical?
As a linking adverb, we usually put a comma before and after besides in writing:
Ex. I don’t think going for a walk is a good idea. It’s quite cold, and, besides, it’s getting late and we don’t want to be out in the dark.