There are several ways to setup FTP on your home Mac. I'll cover the 2 most common ways I would consider for doing this myself. Below I will describe both methods (A and B) and how to access them (C):
(A) I'll start with the native way. There is already built in basic FTP capability within OSX Tiger and Leopard. It does not give you much control other than to turn it on or off, using defined local accounts via the sharing panel in System Preferences (Leopard) under File Sharing. By default uses the standard FTP port of 21.
(B) That said, I would much rather use a dedicate application such as PureFTPd Manager. It's a secure, production-quality and standard-conformant FTP server for Mac OSX. download it here. Gives you full user control individual of system user accounts, as well as supporting SSL Encrypted Logins, Virtual hosts, Virtual directories, Time/IP restrictions and much more.
(C) The last part is how to reach your computer from the outside world, especially if you have a dynamic IP address. You will need to create a hostname that points to your home or office IP address, providing an easy-to-remember URL for quick access. You will use DynDNS.com to accomplish this.
Sign up for the free service to get a domain name. You can also download a small client tool from DynDns which sits on your Mac and sends updates to DynDns letting it know where you are. So when you type in the domain name provided to you by DynDns you get taken to your server.
One last tip. For security reasons, you should consider not using the standard port 21 from outside your network. Use port forwarding on your router to hide from standard hacking port scans.