Im on a diet and excercizing, looking to lose a little weight before summer, ive heard that your breast size may decrease when you lose weight, is this true? and are there any ways to prevent this?

Nope!
There's fat in your breasts, and when you lose weight, you lose it in your chest too.
There's really nothing you can do to prevent it.

Different people lose weight in different areas first. You may not lose any weight from your chest or that may be the first place you notice it!

I have read that doing chest exercises can keep them from getting saggy after losing weight. Such as push ups and weight training exercises.