Move: W A S D
Look: Mouse
Pick/Shoot: Left Click
Freeze/2nd feature: Right Click
Reload: R

Hold Q to open up the menu then click on a object to spawn then you should have a prop spawned on where your phys gun is pointing.

Top Tab
Topics of showing you what items you selected to spawn like cars, entiitles, props, dupes, saves, etc.
Right Side
Showing the tools you can use or settings for (servers, singleplayer, PlayX, etc.)

Multiplayer > (Gamemode) > (Servers)

Sandbox
Default gamemode just for building stuff :D
DarkRP
Just a regular Roleplay Gamemode
PERP
Like DarkRP but semi-realistic roleplay
Cinema
Lets you watch movies with your friends showing the videos in-game! (Adobe Flash Player for other browser required)

The workshop show you files that you can install in your game to make your game more fun like downloading cars, heli, props, effects, handling, skin, players, weapons, etc.

Select garry's mod then click on browse workshop then subscribe + then run the game and the game will install it

There is also tools that you can use! The weld tool lets you stick props together. The axis tool lets you place a prop that you can spin! The light tool just spawns lights. You can always download tools from the Garry's Mod workshop

Weld
About: This tool lets you stick 2 or more props together
How to use:
Left Click: Weld
Right Click:
Un-Weld: R (Be sure you point at a prop with your weld gun to un-weld the proper prop)
Axis Tool
About: This tool lets you make props able to spin!
How to use:
Left Click: Axis Item
Right Click:
Un-Axis:
Wheel Tool
About: Sort of like axis tool except you spawn wheels can able to spin them by pressing a key.
How to use:
Left Click: Place Wheel
Right Click: Change Settings of The Wheel (Changed settings will be appied if you change it in the tools in the spawn menu)
Un-Wheel:
Light Tool
About: Just spawns lights...
How to use:
Left Click: Spawn Light
Right Click:
Un-Light:
Lamp Tool
About: Simular to lights but they act like a flashlight
How to use:
Left Click: Spawn lamp
Right Click:
Un-Lamp:

What is this?
If you have another games installed that supports on props and cars to gmod, gmod will allow you to use those items from the game you bought in-game.

How do I mount a game?
In the gmod menu, there you will see a game controller icon in the bottem right. Click that and check mark on the games you like to mount to use the maps, items, cars, weapons, etc.

I mounted this game but it doesnt work on my game.
In that case some Valve games doesnt work on gmod like Left 4 Dead 2, Portal 2, and else others.