You might ask yourself what is Steam, and why do I need it? First and foremost, the only way to gain access to the Source SDK is by downloading the SDK or a Valve game that uses the Source engine. All of Valve's Source engine games are downloaded and managed with a program called Steam. Steam lets you purchase, organize, and update thousands of games, and it's completely free.

In order to download and install Steam, you need to visit store.steampowered.com, and click on the Install Steam button in the top-right corner of the web page, as shown in the following screenshot:

This will redirect you to another page where you can download the actual installation file. After you have downloaded and installed Steam, you need to log in or create a new user account if you do not already have one. All you need is a valid e-mail address in order to create a new Steam account.