'Beside' and 'Besides' are easy to mistake because the only difference is the letter 's'. However, the meanings are very different. 'Beside' - without an s- means 'next to'. For example, Tom is seated beside Alice. 'Besides' - with an 's' - states that something is in addition to something else. For example, Besides math, Peter is getting an A in history.

Examples
Hang your coat beside mine over there.
There is so much work to do besides the normal tasks. 
Come sit down beside me.
Besides potatoes, we need some milk.