Registering with Chess.com
Go on site chess.com
Click play online
You're presented with a mini-window entitled "Play online Chess". It will automatically allocate you beginner status.
Click on Sign up
On the second window, input a username, email address and password.
The next screen asks for the same details, but also asks you to input details shown as an image, to ensure you're not a robot
You will then get an e-mail, showing you how to activate your account
Click on the activate account box
You can start a free trial of Chess.com premium if you wish. I haven't done this, so I don't know the advantage/disadvantages