The console is a command-line interface for the advanced configuration of games.

To enable console in game You need to enter into game options.
Then in "Keyboard" tab press "Advanced" button.





















Small window will pop-up.
There are two options. Check the second option and press OK.








Above steps are showing way to activate console in game Team Fortress 2.
In other Valve productions (like Half-Life or Counter-Strike) console activation method is the same and that option is mostly in same place - in keyboard advanced options.

Open Your game library in Steam.
Find Team Fortress 2, press right mouse button on it and choose "Properties".





















Then click "Set launch options" button and small window will appear.
Put -console launch option in that window and press OK.

To open console You need to press ~ key on Your keyboard.
With tabulator key You can see hints to commands.
To separate many commands in one line use semicolon.
To send commands just press Enter.