I've been in London twice... haha
London eye, the best place to see the whole London, amazing views of the city, i recomend you going at night, is awesome!!!
Madam Tussaud's is really really good, at least i loved it
uhm... Trafalgar square too and then u can go walking from Trafalgar square to Piccadilly circus, definetely, u should go to Piccadilly
And do not forget Tower bridge, is awesome!!!