my daughter has always been a tomboy - but is soo girly. i think a lot of it had to do with having to be rough and beat up two brothers and mum not being arsed to iron frilly dresses when jeans are so much easier.
so listen wisely grasshopper my kids love to pideon hole - chave, goth, punk, emo - all linked to music choice. ( mine are in senior school)
my girl is a self proclaimed EMO, likes the darker side of music, faux punk, bit of rock, my chemicle romance etc etc.
she dresses uber cool when she tries, this includes jeans with statement belt - so she wears a glittery skull belt. with t-shirt.
t-shirts are best if they are teen angst - che guevara, kill the hippies - kind of thing, but equally plain t-shirts will do
so, my advice would be to get a couple of pairs of cheapo jeans - one dark, one light,
spend the money on the accessories - the statement belts. and lots of plain t-shirts from primark.
the other thing which is a must spend is the shoes.
shoes and belt - i swear. so if chavtastic - its trainers, if goth/emo tastic is boots with metal on.
if you have access to a market near you - travel!
my daughter is a complete snob. i took her round oldham inside market a couple of weeks ago and she nearly died. so i told her she better get some jeans for a tenner becuase i'm not spening £40 back home
hope this helps