First, there is some debate about this, but in my, strong, opinion, you are better off preparing some before taking that first test. At least go over the question types. Do some problem solving questions, some data sufficiency questions and some of each type of verbal question so that at the very least you are familiar with how each type of question works and won't have to consider how to even go about answering them. That way you will get a better read on where you really stand, without your results being colored by your being totally new to the game. I would even recommend doing a math review, if that's appropriate for you, and overall preparing for a week or more before taking that first practice test.
As far as which test to use for your first one, the answer is a tossup, I guess, although I think you would be better off using a GMAT Prep test first. The Manhattan tests are pretty good, but don't quite duplicate the look and feel of the actual test. So to best see where you stand and to get the most accurate sense of how the test is, you would use a GMAT Prep test. It is a tossup though, and you may even want to save the GMAT Prep test for later, as there are only four available.
As far as analysis goes, in my experience, one can pretty well tell by going back over the test results what one needs to work on.
So to a large degree the choice depends on what you know about yourself and how you work best.
