Instructions
Open Steam on your computer and login.
Click "Library," and select "Games" to view all of your installed games.
Locate "Team Fortress 2" in the list, right click on the title and select "Properties."
Click "Set Launch Options," and in the Launch Options text field remove the "-windowed" string of text if it is present. Type "-fullscreen" in its place. The next time you run Team Fortress 2 you will see it open full screen.