You must first know the name of the system you want to connect to.
We'll use ftp.uu.net as an example. On your system, type:
ftp ftp.uu.net
(the actual syntax will vary depending on the type of system the connection's being made from). It will pause momentarily then respond with the message
Connected to ftp.uu.net.
and an initial prompt will appear:
220 uunet FTP server (Version 5.100 Mon Feb 11 17:13:28 EST 1991) ready. Name (ftp.uu.net:jm):
to which you should respond with anonymous:
220 uunet FTP server (Version 5.100 Mon Feb 11 17:13:28 EST 1991) ready. Name (ftp.uu.net:jm): anonymous
The system will then prompt you for a password; as noted previously, a good response is your email address:
331 Guest login ok, send ident as password. Password: jm@south.america.org 230 Guest login ok, access restrictions apply. ftp>