A mix of body weight exercises would be a great place to start, and if that does not help after a month or so, you can look into weights. For Core stuff, focus on flutter kicks, crunches, and front and side planks. Look around on the internet for some other exercises, there are a ton of them. For upper body, you really only need two exercises to get a decent start: pull-ups and push ups. There are plenty of grip variations you can do on both to hit different muscle groups. After you build a good base with that, start looking into some weights if you still want to get stronger, but some of the fittest people in the world use chiefly body weight exercises. If you do end up weightlifting, I recommend looking into some Olympic lifting instead of just the usual bench/bicep curl/row type splits. O-lifting is terrific for your core strength, especially your back, but make sure to find someone who can teach you proper form if you decide to go that route.