This is something everyone should do. It's easy. Just found a counter. You can use the bathroom counter, or something. Put your hands on it, and lean so you're straight, and bend your arms and push your chest in, and then bend your arms back. You should feel it in your chest if you're doing it right. You can look up tutorials on how to do this. It's just easy to do, and you can do like 15 of them at random parts of the day. It makes a physical difference for sure.
There's a bunch other swift easy exercises that make a difference too. I just think you should start doing these multiple times a day. Like... anytime you go to the bathroom, or something.
And when you've gotten good at counter push-ups, you can go to Lv.2, which is bathtub push-ups! This way, you're at a lower angle. It's a bit harder, so you need to gain EXP with counter if you're weak. And then Lv.3 is floor push-ups. But I think Counter is really good and easier.
So start doin' 'em!
EDIT: If you can't do these because you've got a physical condition... I'm sorry... I've got some myself (metal rods in spine) but these, I learned to do just recently, and they're helping me a lot by making me stronger so I don't deal with a lot of pain.