First, you will need a Second Life accound. If you don't have one yet, click here and go create your account and download the viewer.
Second, jump over to Messenger, to ease that process don't hesitate to use this direct teleport link.

Third, locate the huge SL-Messenger panel on the island and left-click on it. When you get a blue pop-up click on the "Keep" button.

Fourth, open your inventory, "SL Messenger" folder, right click on the item named "Sl-Messenger.com" and select "wear".

Fifth, the watch will start talking, open your history and click on the registration link it provided.

Sixth, choose your login, and input your email. If possible try to use the same email as the one used when registering your Second Life account. You will be emailed shortly with a password.
Congratulations, you're ready to go ! Time to go back to home page and login!
Go through our video tutorial if anything seems unclear, if that's not helping, check out forum or help pages or mail support for help!