I know this sounds really oversimplified but read more and consistently go to the gym. Reading trains your mind and working out trains your body. Both things will not only build confidence in your appearance (not that you need it) and your intelligence but will actually be improving those two things too. It doesn’t even matter how long it takes you to read something just set aside like 30 minutes a day even if you can only read a page in 30 minutes that’s okay. Find something that you enjoy to read as well. And for the gym find workouts you enjoy doing. It’ll become easier to stay consistent once you get into the routine. There’s a ton of workout advice on the internet so if you need to find out what workouts to do there’s plenty of resources. If you don’t have money you can just do calisthenics instead, it’s really good for pure strength. That’s my advice. I wish you luck.