There are lots and lots of fitness "experts" on YouTube who will show you a thousand different exercises, and a million ways to do them wrong. There's so much bad advice out there, you'll get better exercise by taking a nap - at least you won't hurt yourself.
But there are gems in there. One of the best is Jeff Cavaliere of AthleanX. He’s obviously in incredible shape, and he has a great emphasis on bodyweight exercises with minimal equipment. He has very clear videos with sensible explanations, none of the mumbo jumbo of lesser competitors. His emphasis on form and technique get you great results while limiting injuries.
You can’t go wrong watching any of his videos, but the place you should start is simple: standing with good posture.
You might think you’ve been nagged about this enough your whole life, but you probably don’t appreciate the core strength and attention to body dynamics that it takes to really stand tall. Watch this and see, then go on to the many other great videos at AthleanX.