What Do You Mean When You Say "Beside" or "Besides"?
Let's start with the easiest situation -- which is when we use beside to talk about our physical surroundings. In such cases, it means beside in the sense of next to or at someone's side. When this is the meaning you have in mind, there is not a one-word equivalent in Spanish for beside, but rather there are a couple of phrases: al lado de or a mi lado. The word lado means side, as in the physical side or border of anything, living or not. It is related etymologically to the English word lateral, which should make it easy to remember. Notice the possessive adjective in the second phrase? Instead of mi (my), it could be tu (your), su (his, her, your -- singular or plural -- or their) or nuestro (our) -- depending on whose side you're talking about. As the second example below shows, sometimes we use side in a figurative way, that is, metaphorically, by saying that someone stands with me -- in agreement.
Su casa está al lado de la estación (His house is beside/next to the station).
Todos mis amigos están a mi lado (All my friends are on my side).
One Spanish word that answers the call of another use of the English word besides (note the plural in this case) is además. When we use beside with the meaning of moreover, or furthermore, the Spanish word además must be used. It is a great word for bringing up a new subject to augment or emphasize what one has already been saying or to change the direction of a conversation.
Además, no me interesa meterme al agua a causa del frío (Besides, I'm not interested in getting in the water because of the cold).
Finally, when de is added to además, the idea of beside becomes aside from or in addition to -- a definite way to augment or emphasize the topic of a conversation.
Además de estar enfermo, Juan tenía que terminar un proyecto (Aside from being sick, Juan had to finish a project).