Hey,
I am a student and I started with the no. 13 and now I'm playing the no.10. Before you start with the sinfonias you should play some inventions, have you done that ? When you start reading a sinfonia you should be careful with the fingering because it's really important to the middle voice. You have to give the same importance to all 3 voices and you must be able to play them alone. The fingering can be awkward some time but you will get comfortable with it, eventually.