Exactly. My bad posture got fixed immediately. Good posture is defined by core muscles (what you have around your waist and a bit above), and contrary to what m

Exactly. My bad posture got fixed immediately. Good posture is defined by core muscles (what you have around your waist and a bit above), and contrary to what many people believe, straight back and posture is much more a function of strong belly than back muscles. But as name suggests, core is a set of them, truly a ring around the whole waist. I personally do squats, deadlifts, 3 minute planks at the very end of workout ('finisher') and other exercises where core is a secondary, stabilizing muscle (pullups, running etc). For start I would recommend something with body weight (pushups, planks), simple and easy to do, and focus on many repetitions. Much safer than trying to push to your max and injure something in your back with long recovery. It's a mistake in training to focus only on one part of the body. Over time developing all muscles helps tremendously with any activity/sport.