In the Steam application:
You can create a new Steam account by clicking on the following link and downloading Steam.
http://store.steampowered.com/about/
Then click “Create New Account” at the bottom of the registration box.
Via the Steam website:
Visit the following website:
https://store.steampowered.com/login/
Then click "Join Steam".
Fill out the form, and then click "Create my account".