Umm... well... yeah, get a bra and, though I've never done this myself, my cross dresser friends used to use nylons, doubled up and filled with dried peas, popcorn, or rice. Seemed to work well, I guess.
Personally, I just put on a cute dress and walked out the door my first time out. No one said shiz. Presenting female doesn't necessarily mean boobs, hon. It means being yourself. For my 2ยข, I'd avoid getting all wrapped up in the 'OMG boobs and butts and face and and and and annnnnnd' stuff. Just be you, make sure that you're as hairless as you want to be in the places where you want to be, get cute, go out. I've always imagined using boob fillers as something of a horrible idea just because you'd have to come home and then take 'em off and then... *bam* boobless. Even after HRT, I only have an A-cup but I really don't have a problem with it, I just make sure that I'm being myself and everything seems to work out.