From the top right, look for the vChat icon on the top right next to the Bell.
Click on it and you'll see a screen that tells you to create a username.
Hover over the 'info' icon to get more information on what makes an acceptable username.
Here are the requirements:
- Must begin with a letter
- Must not begin with the symbols: .(dot) - (dash) _ (underscore)
- Must contain between 3 to 22 characters (including numbers)
- Must be in lowercase
If your username is accepted, you'll see a green tick appear.
Hit 'Go' and you're ready to start chatting!