You just bought Garry's Mod. You have no idea what to do. But this guide teach you how!

*You should be on the main menu of the game.

*Find the tab that says "Start New Game" and click on it.

*There should be two maps: gm_construct and gm_flatgrass. Click on any of them.

*Then close to the right down hand corner, you should find a button saying "Play Game", and click on it.

*You should end up on whatever map you chose. You should get an achievement that says "Play Singleplayer".

Alright, have fun playing Singleplayer!