Barbell Squat

Pick up the bar and put it on the medium part of your back. Stand with your heels outside hips, toes slightly turned out to the side. Keep your chest up tall and brace your abs. Start pushing your hips back and down, like you want to sit down in a chair. Inhale on the way down and exhale on the way up. Make sure you keep the weight in your heels and don't let your knees go over your toes. Keep your chest up and your shoulders back the whole time.

It works several major muscle groups, plus if you do it fast it can also significantly improve your cardio.