I watched tutorials on youtube, like TopHatWaffle, who was already mentioned in this thread. And of course, was digging around in someone else's maps like mg and ze. Doesn't take a genius.
But if you are aiming at creating high-level maps, you might wanna consider working outside Hammer. Take stuff from other games, like models, textures, particles and convert them into CSS/CS GO by other programs.
I mean, it's hilarious to play on old maps, for example, which are packed with countless (!!!!!!!!!!) crates and painted with sad HL2 textures. Try creating something fresh.