well how far have you gotten?
did you install subsonic on xp? can you go to
http://127.0.0.1 in a web browser and get to the subsonic server? that's a start.
Is your home computer always turned on? It will need to be to connect to your server from your phone. Next you'll have to see if you can connect to your server from the web. Go to
http://whatismyipaddress.com and plug that (your external IP address) into your browser (http:// then the number that whatsmyipaddress gave you).
If going to 127.0.0.1 works but your external one doesn't work you'll have to edit the settings in your router to forward port 80 (should be the default port in XP) to your computer (you might also want to assign an ip address from your router to your XP computer so that doesn't change if there's a power outage or you restart your computer).
If you can get to your subsonic server from your external address that is the address you'd put into your phone. You might want to try going to
http://dyndns.com to get a domain that will forward to your computer in the event that your external IP address changes and you'd use that for the address in your phone (subsonicserver.dyndns.com or something like that).
So, how far have you gotten and what is/isn't working?