Hi Ashley. Be mindful of early reports... I think that swelling and sensitivity of the nipples is the first thing to happen. It's nice, but not breast development in the strict sense.
The "buds" ( lumps directly under the nipples ) are the first signs. For me they started at about week 6, starting at the size of peas. By week 9 they were walnuts. At this point I cannot go bra-less. The nipples are almost always erect, and too sensitive to rub on any fabric. Also my boobs hurt like hell with any amount of pressure, so protection is good.
The breast shape itself ( fatty tissue ) didn't start until about week 10, and I exploded quickly over about two weeks into an A - cup. I'm at week 14, they've just started hurting again, suggesting another growth spurt is coming.
While a blessing, can be hard to manage before going full time if you're a fast developer. I've very little body fat and a ribcage designed to push them out, so they're prominent features on me at even A cups. I'm glad it's winter where I am, because I can't unzip my baggy hoodie any more without them being on display. And tight fitting clothing is not recommended...