Out of my head:
when installing LW, it asks which directory you want to share. You still can change this setting afterwards.
Then you should be connected to the internet. If you have a fixed IP address you can mail this to your friend, and he can immediately connect to your machine (I think it was under 'Connections' where you can enter a specific IP address to connect to).
If you dont have a fixed IP you need to find it out first (ifconfig on proper systems, netstat -r or winipcfg on these windoze-playstations) and tell it to him. (We hope that there is no NAT in between)
Of course he can always try to connect to your machine by searching for this song....

HTH
Axe