This is very compilcated. First, you need to turn your computer/laptop on. To do this you need:
1) a hand
2) a brain
3) a button
4) two eyes

First, carefully press the power button on your computer/laptop/potato. You need to make sure you don't press the button too hard or you will break it. Second, you need to wait for it to turn on. To do this you need to simply wait. After a few seconds, your computer should be turned on. Next comes the tricky part: logging into your account. There should be a little arrow on the screen. Use the mouse to move the little arrow onto your user picture, then press the Left-click button. Then click the Steam icon on your desktop. When you have loggen into your Steam account, click on the button that says "Garrys' Mod". Finally, you need to click the 'PLAY' button. This will launch Garry's Mod and you will be able to play.