When I started out as a mapper, like a lot of people I turned to YouTube, and found 3kliksphilip. There's a lot of hate surrounding this guy, and I admit that as I become more experienced I find better ways to do things than he does, but he helped me start mapping and for that I am grateful.
There are, however many other places I've found that can help with mapping:
Half-Wit 2 has a wealth of tutorials, from beginner to advanced, and it serves as a good place to get started.
Interlopers also has a lot of tutorials, covering slightly more varied topics than Half-Wit, but less geared towards beginners.
The Source Wiki serves as a good place to go if you need more details on specific entities, but is lacking in tutorials.
Finally, Facepunch's Mapping board, specifically the Mapper's Encyclopedia thread, is a good place to ask questions. You can also post works-in-progress in the map pimpage thread.
There are many other resources out there, but these are the ones that have helped me the most.
Good luck and happy mapping!
