You can join Steam from the Steam Official WebSite.
->From Steam's official site, go to the upper right corner of the screen and than 'install steam'
->Click the icon to install the program.
->When the steam program installation is completed, the 'Login screen' of the steam launcher appears.
If you already have an account, than you can log in with an existing account, but if you do not have an account you can create an account there