gap year to be spent in China.

Hello,
My Girlfriend (German) and myself (Belgian) want to go and travel around in China.
We would fly to Beijing, then go to Xi'an and then later onwards to shanghai. Do we need to book all our hostels/hotels already?
We would also like to see more but don t want to have it all planned. Do you have to book everything before even if you want to travel around for about one month? Or is an onward ticket and some hotels sufficient?
We would apply in Bangkok for our visa. How long does it take and do we need something extra?