There are three things you need in order to FTP to a computer at home (or anywhere, for that matter):
1) A path to the FTP server (IP address/domain name)
2) Port 21 open on the FTP server
3) FTP Sharing enabled in the system preferences on the FTP server
Typically, home-based internet access (DSL, Cable, etc.) have a dynamic IP scheme, meaning your IP address changes every so often... so if you know your IP address now, you can't guarantee that'll be your IP address later today or tomorrow. In this case, you can sign up for a dynDNS.org account, which will allow you to access your computer with a name like "homeftp.dyndns.org", and dynDNS.org keeps track of your ever-changing IP address. Sign up there if this describes your setup.
Next, you need to open port 21 on your FTP server. If your computer is connected directly to your DSL or Cable modem, head on over to the Firewall settings on your FTP server and ensure that port 21 (or "FTP Server") is selected/enabled/opened. If you have a router in between the modem and your computer, then you'll need to read up in the owner's manual about how to do "port forwarding" -- then, open port 21 and forward it to the local IP address of your FTP server.
Last, enable FTP sharing in the System Preferences > Sharing section.
That's it! You'll be able to connect from afar by launching your favorite FTP client program and entering either your home-based FTP server's external IP address, or your dynDNS account domain ([something].dyndns.org) using your Mac OS X username and password on the FTP server machine.