Download putty from here.
Open it up and click the redbrick button.
Type in your username.
If you make a mistake typing in your username while using SSH (ie when you click the big "Redbrick" button to log in) you'll have to restart as you can't go back to edit it.
Type your password (note, nothing appears when your doing this. That's delibrate. It's a unix thing).
When you're logged in, type help.
Or if you want to dive right in, type chat.
Once you've finished with all that and find you want to leave (!) you can type logout to end your session.
PuTTY Manual - A more indepth tutorial on how to use PuTTY