There are a few ways to do this. You need to find out what your External IP is
Your IP is probably 192.168.0(maybe a 1).1 or something similar, it depends on your router.
192 is typically always an internal IP address.
You need your external
Don't post it here, but go google up some sites, and then let me know when you have it.
Also, don't expect your friend to be able to connect. It might be possible, but I know in the US here, a lot of ISP's block incoming traffic to home addresses. Not always, but it does happen.