This is the easiest part of this guide here. You simply have to click to Library button at the top bar, find Garry's Mod in your library and click Play button respectively. And you shall be in the main menu.

Ok, now that we are in the main menu. You have to load a map to be able to jump (basically). Now, you only have to turn on the developers console by pressing the "~" button located under the "Escape" button which is at the top-left corner of your keyboard (probably). Then you just have to type "map gm_construct" and press "Enter" on your keyboard.

press space (that wide a$$ button on your keyboard)

ok, you're done, bye.