My DD is similar - 5ft 9, size 12 top with 34e boobs, size 10 bottoms but fairly powerful thighs.
The single thing that makes the most difference is a properly fitted bra. My DD was extremely reluctant to be fitted and had to be bribed and cajoled, but recognises the benefit now.
We get jeans from top shop as they reliably have a good choice in 34 inch inside leg (online but order a few pairs to store for a try on). She also favours tops from top shop. Generally things with a lower neckline but actually in a properly fitted bra she looks good in a high neck top anyway as she is 13 so they tend to always look good.
I think a big h and m is a good place for basics tops, some things are randomly 100% cotton. Also we've had more success with little dresses from there as dresses from Hollister etc don't really fit her.
She wears a lot of baggy casual wear from places like Jack Wills and Pink. It depends on the girl and the style locally, but she and her friends seem to hang out all dressed in sweatpants etc.
It's hard to be a self conscious teen and particularly if you stand out through height or whatever. I make sure DD has the right things re trainers, boots, make up etc so that she doesn't feel any difference in those areas. For her that is for example Nike roshes, adidas superstars, uggs, Mac and urban decay, but I'm sure that varies by taste and school. Can you ask her what she wants and give her a boost with the right trainers or whatever.