The Garry's Mod Wiki site includes tutorial articles. The game modding community GameBanana and mod database ModDB.com also host Garry's Mod tutorials. YouTube channels dedicated to the Garry's Mod scene, such as the PlayingGMod channel, often upload tutorial videos. Individual modders also sometimes upload YouTube videos explaining their modding techniques.
The Garry's Mod Wiki has a Getting Started section that includes up-to-date tutorials suited for beginners. Note that many Garry's Mod tutorials are outdated -- some of the ones on ModDB are no longer relevant.
Popular YouTube users who upload Garry's Mod tutorials include kitty0706, JackkTutorials, JackpodyDK and "Tutorials and Such." Remember that the oldest tutorial videos on YouTube are unlikely to be compatible with recent versions of Garry's Mod.