If your a girl, it can be Justice, Old Navy, Aeropostle, Abercrombie, Hollister, American Eagle, etc...
Aeropostale, American Eagle, Justice, and Hollister are some of the many places
FashionTiy
zumiez and tillys
Onverse is a good choice!
I am a tween and i always go to Aeropostale, Hollister, Abercrombie, and Abercrombie and Fitch. Some of them are pricey but you can get some really cool stuff from the sales rack if you can't afford it (like me, sometimes). those are the stores were most teens and tweens shop, and its totally appropriate!
Gap and Gymboree are good stores for little girls clothing.
Good online clothing stores for teenagers would be a teen based store.us A store that specialises in especially for teen style. Stores such as wetseal, rue 21, forever 21 arsenal stores teenagers would be interested in.
facebook is one of the good ones myspace probably not
I think some good virtual game sites for tweens are fantage, movie star planet, club penguin, and more.
Try sites like eBay or amazon. Some discount stores have good deals. Check for sales at stores. The swapmeet might some times have cheap cute girl clothing.
JC Penny's, Macy's, Target (not fully clothing), WalMart (not fully clothing), Nordstrom, and many other stores.
In the UK the stores Pumpkin Patch and Mothercare specialise in children's clothing, however, one can also find a good selection at Next, Asda, Tesco and Gap.