Remember girls are human beings too. They have their dreams, hopes, fears, aspirations. They get scared, nervous, shy or insecure sometimes. :) Just like boys do.
They are sometimes insecure about the way they look, so feel free to compliment them. :) (But only if you mean it.)
Approach them as human beings. Talk to them as you would with any other person. Do not be afraid of them. People are often afraid of things they do not understand.
Look them in the eyes, and listen what they are saying.
Do not belittle them.
Look them in the eyes, not at their breasts.
Why not? How would you like if someone stared at your crotch the while you speak to them.
Besides, it erases everything they ever did in their life and equalizes them with a sex doll. A sex object.
Imagine if what ever conversation you started someone comes to you, and says: "Yes, yes, don't you worry your pretty head about it" :)
Pretty infuriating, isn't it ? :)
Their breast are part of their body, not some fashion/sexual accessory they put on when they go out.
They live with them 24/7.
They are not 8th world miracle.
Although there are push-up bras ... :)