What I'm trying to do is set up a SSH key that only gives access to certain directories, for security reasons I don't want it to have full access to my SSH server. I already have the ability to access the directories I need over my local network (right now using SMB, although I also used UPnP for awhile). I need, however, to be able to access those files securely over the internet from both Ubuntu and Windows machines.
I'm somewhat new to SSH and not sure what the best approach to solving my problem is. If anyone knows how I can do this or where I can find a detailed tutorial I'd be grateful.

And as always if anything is confusing or if there are any comments or corrections please let me know.