Check your posture this way: stand relaxed and face a mirror. Do you see the back of your hands or the sides?
If you see the backs, that means your hands are internally rotated and your shoulders are likely rotated, too. Now, rotate your hands and arms outward, away from your body. Feel the squeeze of your muscles between your shoulder blades? That’s what you want! Those muscles need to be strengthened.
My favorite back exercise is the self-row, also called the supine row. I often do this with clients first thing to work their weak rhomboids, which they invariably have from sitting at a desk all day. This improves posture, strength and metabolism and relieves back pain.
Here is a demo with suspension straps. You can also use a squat rack with a bar, two chairs with a broom stick, or the edge of a table!
Lift your body like it’s a straight board, a plank. Or lift your body with your knees bent, feet on floor, if you can’t lift fully as a plank.