How do you open devolpers console in team fortress 2

By pressing the tilde key (~) on the keyboard.

You need to have the option actiavted first.
From the game menu, select options, advnaced options, there will be a check box for enable developers console.

This option will not be there if you are Free-To-Play.

You can enable this option by buying anything from the Mann (in-game) Store.