Connecting domestic flights within China — for example straight after your arrival — can already be booked through your travel agent back home. For a connecting domestic flight after landing in Beijing or Shanghai, you'll need to collect your luggage and check it in again; it is not automatically transferred.
Domestic flights can also be booked at local travel agencies once you're in China, and most agencies have English-speaking staff. To be safe, book a few weeks ahead, though tickets can often be arranged locally on short notice too.
