Skills you should have for the midterm:
Review Book You Should Get:
Barton's AP Chemistry, 7th Edition: this has been updated for the new AP curriculum fairly well. I highly recommend buying this and working through the chapter quizzes and, eventually, the full length practice quizzes.
Links to Online Resources:
AP Chemistry NMSI : notes, practice tests with explained answers, advice. Highly recommended. Not 100% updated for '13-'14 curriculum, but still good.
Learnerator: free online multiple choice questions, scored as you go. Quality of questions is highly variable, with some being misleading or of very limited value (have heard reports of some outright incorrect ones, but can't confirm or deny). Can't access super-difficult ones without paying. Still useful, but be aware of its limitations.
Chemmybear: study guides, notes, worksheets, links. Also not completely updated, but much of it still relevant.
Chemistry Olympiad: old exams from this event. Does NOT follow AP curriculum, but much is the same. Generally considered to be harder than the AP test.