My 12 year old niece is blossoming into a young lady and recently got measured for her first bra. Coincidentally it turns out she is the same size as me and so I felt compelled to offer my lingerie wisdom. And if you, like me, are not in the high street 32A to 36DD range you're also going to need to look a little harder for a bra that fits.
If you're not very well endowed it's easy to start looking for bras at shops with girl's clothes ranges such as New Look. They're good for when you're looking for a t-shirt bra with the additional benefit of being a little cheaper. Unfortunately they also tend to have little monkeys or cutesy cupcakes all over them. That's when M&S is very useful as they have the girl's Angel range but also stock more grown up designs in all sorts of sizes and patterns.
M&S is an obvious choice and I haven't really come up with anything profound but the great Marks and Sparks deserves every credit. I can't tell you the number of times I've been to specialist lingerie shops and not found my size. These shops even have little labels on the bras which tell you all the sizes they come in, 30B not being one of them. It really gets on my tits. I get particularly annoyed in La Senza and especially avoid the celebrity ranges.