On your router you would need to:
Open TCP port 1723 for PPTP or UDP port 500 for IPsec.
Also open IP port 47 for PPTP and 50 for IPsec.
For windows VPN/RAS servers you will also need to enable GRE to be passed through.
You will need to then direct inbound packets on these ports (and protocols) to your VPN Server/appliance’s IP address. If you do not have one Microsoft includes components that will let you setup one with windows NT/2000/2003.