How do I get a flat stomach?

To get a flat stomach you need to eat at a calorie deficit to lose fat. Simple.

However it is also a very good idea to incorporate weight training and exercises that work the abdominals (squats, deadlifts, leg raises etc) and the rest of the body.

When someone says something about lifting or lift heavy they are talking about weight training which includes body weight, dumbbells, barbells, etc.

Heavy is also relative. Heavy is what is heavy for you.

http://www.myfitnesspal.com/topics/show/1080242-a-guide-to-get-you-started-on-your-path-to-sexypants