My casing has absolutely no fans in it, besides the cpu and graphic card fan. I'm using this cheap casing, except there's a hole in the side of it unlike the pictures.

I was thinking of getting a side fan, since there's a hole on the side of the casing..but what direction do I put? Blowing in or out? I heard it's better to have one fan in and one fan out but there aren't anymore holes in my casing...if that's better is it possible if I removed those empty slots in front of the casing and try to "stick" a fan there?