Descartes' Meditations are meant to be read through slowly meditatively.
He is concerned with knowledge and certainty in ways that Plato,Aristotle and earlier philosophers are not.
He uses doubt to discover if anything is certain.
A question to ask is regarding his starting point. While earlier philosophers will start from sense experience as evident and the basis of our other knowledge, Descartes starts from the internal evidence of his thinking (a step that would have come only later for someone like Aristotle or Aquinas); the problem with this approach is that it introduces a gap between the way the world appears and the way the world is and makes philosophy subjective; that is I can't really be sure that what you are perceiving as real is what I am perceiving; thus I can only be 'certain' of my own internal ideas. As you read through Descartes can you find places where he seems to be stuck in this internal 'bubble'? do you think one's search for certainty can backfire so to speak?