There is usually a command that you type into the console (the command prompt type of thingy). Check your game company's website for the commands...
For halflife:
You can access the game console with the tilde (~) key. If that does not work, do the following:
1. Right-click in the middle of the desktop and choose New then Shortcut.
2. Click on Browse.
3. Choose the C: drive for Look In at the top of that window.
4. Double-click on the Sierra folder and then on the Halflife folder.
5. Then double-click on the hl.exe or hl file listed there (it will appear with the
Half-Life logo icon).
6. Click on the Next button and type Halflife Console.
7. Click Finish.
8. Right-click on the Halflife Console icon that is now on the desktop and choose
Properties.
9. In the Shortcut tab at the end of the Target file add -console (it should read
"C:\Sierra\Halflife\hl.exe -console"
10. Click Ok and use this icon to launch Half-life with the console enabled.
Now you can use the "~" to access the console as needed.
**Taken from Sierra's website
type in: host_speeds 0
enables display of current FPS on-screen