Behind the lens. I still suck but I've gotten better. It's like anything else, practice.
What Tim said.
If you want to learn to take good pictures, keep a notebook in your pocket and write down the settings (f-stop, shutter speed, ASO) of every picture. Then, when you look at them later, you can learn what looks good and what doesn't.
SHOOT A LOT I mean like hundreds of shots. Back when I used film, I would expect one good shot for every other roll. Digital makes the learning curve much steeper. Don't worry about post picture processing (Lightroom/photoshop etc) until you get the in camera stuff down.