Lots and lots of pushups, bench press, incline bench press, chest flys, and a clean healthy nutrition low in fat high in protein.. Do your training with body weight, medium weights and heavy weights. If you have alot of body fat, Cardio will too..
"Harden boobs" with exercises?
There's no such thing. You can do a boob job.
You can do like the guy said, but this will help to build your pectoral muscles not your boobs. They are different things.
You can decrease body fat and increase pectoral muscles, it will "hard your look" but you will have smaller boobs, because boobs are fat, less fat, less boobs. Is that what you want?
This gets asked every week. And, like Florence says, you can't harden up boobs. the actual breasts is mostly fatty tissue (and a little glandular tissue too).
What works for men is what works for women - and there's a TON of chest workouts you can do - for the muscle underneath your breasts.