Firstly open up Garry's Mod to the home page and click on the option button, the box below should open.

Secondly, go down to the button "Advanced..." and click it.

A grey box should appear with the options:
"Fast Weapon Switch"
"Enable Developer Console"

You must click the button "Enable Developer Console" so the small white box should have a small green tick like below.

Finally press the Tilde (~) or the Grave Accent (`) - otherwise, press the F10 key - and the following should appear:



To those who are confused on what I mean or have no clue where the tilde key is, it should be in the area where the picture, if not you need to hunt it down or find help.