Hi Venus and welcome to 3FC!
Boobs are mostly fat and a lot of us lose fat from the head down, so the boobs are often the first to go. As a result, unfortunately we can end up with saggy boobs from the loss of fat and excess skin.
Chest exercises will strengthen the chest muscles that lie underneath your boobs. Unfortunately, they can't make excess skin shrink or give us perky boobs (rats!). A breast lift is the only way to get perky boobs after weight loss, especially if you've never had them before.
BUT! Keep lifting weights while you're losing weight! It will increase your metabolism and help you to look your best when you reach your goal weight. You may not have perky boobs, but you'll be tighter, smaller, and more toned than someone who doesn't work out while they're losing. Guaranteed!
For more info on skin and weight loss, check out this thread from the Maintainers Forum - Weight Loss And Skin FAQs