How do you get the cheats up for left 4 dead PC

You need to enable the developer console, which is an option under "Keyboard Controls" - "Advanced". You can only access the console through the pause menu.

Here's a handy website with many console commands:

http://www.gamefaqs.com/computer/doswin/code/937406.html

Note: You cannot use cheats when playing online. You will be kicked and banned from game servers for cheating in the console or hacking the game. Any active console cheats will also deactivate L4D achievements.