Steam is Valve's software distribution platform. Once you install Steam and get an account, search for "Team Fortress 2" using the search box at the top-right. Click Team Fortress 2 and then Play Game below the screenshots to start the download. You can see the status of the download at the bottom of the screen.
Once Steam finishes downloading "Team Fortress 2," click Library on the banner at the top, and choose Games. Select Team Fortress 2 from the list on the left, and click Play.Learn more about Video & Online Games