Instructions
Double-click on the "Garry's Mod" shortcut in your Steam library or on your desktop. Steam is the program from Valve which players must use to purchase and install Garry's Mod.
Click on "Play single player" if you wish to start the game in single-player mode. You can also select "Play multiplayer" if you wish to play with friends over the Internet.
Import elements such as characters or objects using the tool gun, which lets you import, place and modify elements from various Valve video games.
Use your mouse scroll wheel to select the camera tool, which allows you to capture screen shots of the scenes you have created.
Right-click and move your mouse around while holding its right button to select an appropriate angle and distance for your shot.
Click on the left mouse button to create and save the screen shot.