Lua, especially with Gmod, can be a difficult language to learn considering it's already been built; you'll have to learn the syntax and Variables that Gmod uses. If you're gonna be coding stuff like DarkRP, it's easy as fuck. But if you're gonna be trying to make a new item or even a plugin for Clockwork, it's gonna be hard as metaphorical steel balls.
If you wanna learn Lua just to fuck around and code something, let me direct you to Java programming using Eclipse. You're free to send me a PM and I'll show you how to set it all up and teach you the basics and everything, if you're really adamant on learning Lua, tell me what you wanna try to code and I'll see what I can do.