Holiday in london how much will it costs?

hey, i'm 15, i live in indonesia. in may 2011, i will be going to london for like 4weeks or so(homestay). but i don't know how much it will cost and where to, like the airfare, restaurants, shopping, food, etc. keeping in mind that i would be living as cheaply as possible. and i also don't know what to wear in london, could anybody help me? thanks.