Starting Out With Ettercap
Note: This tutorial assumes your using a linux or unix system (and you should be if you do anything involving security) as Ettercap's GUI is simplest to run under *nix systems.
The first thing you will need to do is download and install Ettercap.You can download it here. If you want an even easier way to run Ettercap (and an abundance of security programs) you should check out Backtrack - my favorite security distro.
Now lets start Ettercap!
-G tells ettercap to run in it's GTK GUI mode, which is the most useful for now.
If you're using Ettercap on a network with WEP key protection you will have to use the W option, if you want to decrypt packets (you do).
ettercap -W key_length:string_or_passphrase:wep_key -G
where key_length is the length of the WEP key (64, 128 or 256), string_or_passphrase is p or s for a passphrase or string respectively and wep_key is the WEP key. An example run:
ettercap -W 128:p:b3b321e20a1865fed976337d82 -G
The GTK GUI for Ettercap should pop up.