I've a BeagleBone Black doing just that. Seemed easy enough to do.
1) First off check that your pi has an IP address thats in the range expected by the router (/sbin/ifconfig is your friend) - if it isn't you can use ifconfig to reset the IP address. If using dhcp the address should be fine (on my BBB, one adress uses dhcp and is fine, the other one deafults to a BBB setting of 192.168.7.2 which is not).
2) then in your router (which I presumbe is some kind of NAT firewall, that stops everything getting through), you need to set up forwarding. What needs to be enabled is forwarding to the RPi IP address, port 22. Me I forward from a high random number port (something like 12345).
3) Then from outside the router log in using "ssh -p 12345 [email protected]
" - obviously chnging the port to the forwarding number you set up, the user to the user name on the pi, and the ip address to be the address of the router.
Hope that helps.