How do you bind keys on team fortress 2

Changing the controls If you mean simply changing your controls, from the main menu, click options, them double-click on any action and press the key/button you want to use for that action.
Binding New Keys
If you want to bind a key to a special action that is not included in the options menu, you will need to use the Developer Console.

To enable this console, if you haven't done already, go to Options > Keyboard > Advanced > Enable Developer Console.

To open and close the Developer Console, press the tilde key, which can be found to the left of the "1" key and above the tab key.

Type this code into the Developer Console to bind a key to an action:
bind "<key>" "<action>"

Replace <key> with a letter key or number, or if it is a special key like an arrow key or numpad key, a special name must be used.
Secondly, replace <action> with the action you want to happen when the selected key is pressed.

Example
bind "f7" "voicemenu 1 1"
This binds the F7 key to the action that makes your character shout "Spy!".