You have to warm up the whole body since pitching and throwing in general is a full body dynamic activity, but I'm going to speak more about the arm warm up.
What I have tried and liked for my arm warm up(which should happen after you arm up your body in general) is to do the following general warm up
-Med ball throws
And then I'm good to go.
My logic behind this order is to go from a more static limited movement to a more complex and dynamic movement.