How to play Garry's mod

Its basically Half Life 2 with a Physics based approach, you have a Phys gun which can pick up props and entities that are spawned from the Q menu. Gmod also has a lot of custom game modes that you can see in the Multiplayer menu, be careful though a lot of these servers have long first load times to download all the Addons for the server.
Search around and find the right game mode for you!