So you want to start Left 4 Dead 2. These are the steps you have to do:

1. Open Steam by double-clicking the shortcut or searching on Windows for Steam and opening it that way.
2. Go to the "Library" section from the top menu.
3. Locate "Left 4 Dead 2" in your game list.
4. If you have already installed Left 4 Dead 2, click the "Play" button on the game's library page.

Now you know how to open Left 4 Dead 2.
Have fun playing Left 4 Dead 2! :)