Shane's answer is great overall on what to read, but reading your title and question body again... you asked what to skip.
Skip his Biology in its entirety. There's quite a few texts in there. Mostly interesting only on an anecdotal level (nearly all the primary texts here http://plato.stanford.edu/entries/aristotle-biology/). I'm not saying that it's completely worthless -- just that it will only matter if you want to study a particular subfield in history of philosophy of science.
All of some sections of the Politics can be skipped. Particularly, the lengthy discussions of each political system in the Greek world (BK 2 sections 8-12). Some of them are worthwhile so consult a contemporary commentary that should give you the highlights instead of trudging through the descriptions. Also if you will read both Nicomachean Ethics and Politics, you can skip the second half of NE Book 8.