Kara's physique is slim and muscular with the look of someone who exercises regularly but doesn't over do it. What stands out the most are her sculpted arms and toned stomach. As a Super she doesn't really need to do anything to be healthy and in shape. However, we non-Supers need to eat healthy and find time to workout and be active.
To get in Supergirl shape, it would definitely require cardio, total body and weight training and not to mention some form of stretching for flexibility. The goal is to tone up and build strength and if you work out at home (such as myself) investing in a pair of weights and resistance bands would be helpful. However, any move you make is a step in the right direction to getting healthy.
If you're new to working out and getting in shape, start out slow you don't want to risk an injury or a sprain especially since we're not Supergirl.