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.
Something to note if you're logging in using the Computing Buildings (and I assume other buildings) computers:
All the machines in DCU are in a state of deep freeze, this means that anything that you do will be competely "forgotten" when the machine is turned off.
This means that all the software on the machines will not change, so any changes to settings will not be saved.
When PuTTy was installed, our own version with the "RedBrick!" button was not used, instead you'll have to type "login.redbrick.dcu.ie" into the "Host Name (or IP address)" bar and click on Open. You'll then be asked about changed RSA keys, to which you should click "Yes" and then you can log in.
Also the character encoding is wrong, it should be UTF-8. To change this go to Window -> Translation -> Received data assumed to be in which character set: and choose UTF-8
Alternatively, you could try installing PuTTy on your H:/ drive where it will remain saved after you log off.
PuTTY Manual - A more indepth tutorial on how to use PuTTY