Welcome! First of all, what sort of mapper are you?
COMPLETELY NEW!
I suggest watching all of them. The first will get it all setup and will show you how to play your first map in-game. This stage is always boring, because you will be wanting to make the next Dust2 (wouldn't that be Dust3?!). After all 3 videos, you'll be on your way.
VETERAN MAPPER BUT NEW TO CS:GO!
Watch the first video for a no-frills guide to getting your map up and running in the game, as it is a bit different to previous source-based games.
EXPERIENCED CSGO MAPPER LOOKING TO IMPROVE THE GAMEPLAY / LOOKS OF CS:GO MAPS!!!
This tutorial series isn't for you. It shows the basics. Sorry. But hey, you might still enjoy the tutorials! You masochistic person.