Forever 21. Really popular site with amazing clothes and really affordable too :) :
Abercrombie & Fitch. Their prices vary but I think they have a sale or something now xD. :
Princess Vera Wang. It's affordable too and it's for teenager girls clothes plus they have such cute clothes so it's perfect!:
Delias. I don't really shop there, but they have like a 50% off and they have super cute clothes for teenage cheekas :) :
Guess. They're not expensive, but if you pay retail price, then it depends xD. Regardless, check it out. They have the hottest fashions <33
Express. It's cheap & cute too :) :
there's more sites, Juicy Couture, Gap, etcc. there's too many to name xD.
But I hope I helped and enjoy your shopping :D
Try new look because sometimes they have really good sales. But it also depends on what type of clothing you want. Also try tesco and asda and place like that online because they might surprise you but don't go in store because they often don't have as many clothes and also they are very cheap. Places like h&m can also have some nice dresses and shirts. It also depends on your size. If you give me the details i will give you more clothes shops that are better for you.
hope this helped.