So im going to make a file server so I can access files on my network over multiple PCs and I'm lost on what OS to use. Desktop, server or something like pfsense?
FreeNAS, it's based on FreeBSD, it's very stable, easy to use and compatible with all file sharing platforms.
ubuntu server ED/ubuntu desktop(10.04lts)/centos/free BSD/debian5/6/windows web server Any of these will work as a server (windows server requires a monthly licence fee anless you use a loader)
Can I use just plain windows 7? And if so, can I set it up to be accessed through the internet?
Yea you could use straight up windows 7, but you will be installing alot of additional crap to get functionallity such as multi OS connection sharing support and an FTP server for access accross the web.
Look at FreeNAS or Nas4Free. Both are simple, install is a breeze and then bar setting the IP of the network interface all configuration takes place within a browser connection to the box.