What is most important?

The most important items that you will require are a passport, to get you back into the U.S. and a visa for travels in China. You can apply for the passport anytime during the process, however, the visa was somewhat more tricky.

We couldn't apply for visas until we knew where we would be entering China and the dates we would be there. This didn't happen for us until late in the process. We used a service that applied for the visas for us. Of course, if this makes you uncomfortable, you can make a trip to the nearest Chinese consulate and apply in person. It would have been a four day round-trip for us to apply in person. The adoption agency with whom you are working should guide you every step of the way.