VPN setup SBS 2003 isa 2004 Help!!!

I am needing some help being pointed in the right direction. I have a sbs 2003 with isa 2004 and I need to make this a vpn server so I can connect another office to it. I have tried several different things in isa and can’t get anything to work. My private ip inside the sbs server is 192.168.16.2. How do I go about setting this up. I need this to connect to a vpn router ( any suggestions on what to get ) and give about 5 users access to the sbs server. I am having a hard time understanding the isa setup for a vpn server and how to do the site to site vpn. Any help would be greatly appreciated. When I try to add an access rule for pptp access every port scan I do says the port is closed and I am unable to open it.