Register an account
This one is pretty easy. Just click here to register an account, write down your username and password and then come back here.
Download a client
There are many free clients available.
There's a list here:
But we can also reccomend a few:
Set up your account
No matter which client you use, all you're going to need is your username (i.e. firstname.lastname@example.org) and your password (that you used when registering the account).
Put those in and you're good to go.
Add your buddies
Add any buddies in you want just by using their jabberchat address and you're good to go with secure chat.