Beside and besides are sometimes used incorrectly when we make sentences. Although those words are spelt almost the same way, they actually have different definitions and are used in a different way. Beside without 's' refers to the location of something. Meanwhile, besides means “in addition”, “in addition to” or “moreover”.

Using “Beside” in a Sentence

         Beside is a preposition that means ‘next to’ or ‘at the side of’. For example:

                      The barn beside the farmer’s house was falling down.

                      Will you sit beside me at dinner?

         Those sentences would be grammatically identical and consistent in meaning if beside is replaced with next to. For example:

                      The barn next to the farmer’s house was falling down.

                      Will you sit next to me at dinner?

Using “Besides” in a Sentence

         While “besides” can be either a preposition that means “in addition to” or an adverb that means “moreover”. For example:

                      I dislike fishing; besides, I don’t even own a boat. 

         So now, you can improve your English skill a little more when you want to use those words in a sentence. Good luck!

Source:

https://www.grammarly.com/blog/beside-vs-besides/