You need an index file (index.html .htm .php etc)
what Mathiaus said.
Basically, when you go to a directory (say somesite.com or even somesite.com/site) the computer goes "okay, is there a file in here to tell me how everything is laid out? (index file)" and if the answer is no, then it just displays everything in the folder. Obviously if you don't have permissions to view everything in the folder it will give you a 403 FORBIDDEN error.
hope this helps!