Follow the order above Pick easy exercises to start with and when you can happily do about 2 sets in a row of an exercise, try harder ones. Some experts recommend doing abs at every workout. Others recommend doing them however often you do anything else in other words treating them as you would any other body part. Since most people want good abdominal tone more than freaky abdominal size, it probably makes sense to exercise the abs with lower intensity and more frequently, rather than with high intensity and less frequently. Thanks for asking.
Hyperextensions are a good lower back exercise.
Dead lifts, both straight and bent-legged give the lower back a lot of exercise, so if you do them you don't need to add anything else. Make sure you get someone to show you how to do them properly and keep your lower back arched through the whole movement. Lie face down, with your hands touching the sides of your head and your body draped over the edge of the bench. Make sure your hips are supported so your pelvis can't move. Slowly raise your torso to the horizontal position, but no higher.