I am rather concerned here, basically I bought a Chinese Plum bonsai a week ago, well I have done all I was told to do, keeping the soil moist, keeping it in a well lit window sill, however its leaves are browning and dropping off!

I have tried to find advice but it is scarce in supply on this tree and I have no idea whether it is a natural process for it to drop leaves during autumn or not. I have to admit, keeping a Bonsai is much more daunting than I had anticipated! :-(