See Descartes' Epistemology : 4. Cogito Ergo Sum, for some key points, including :
In short, the success of the cogito does not presuppose Descartes' mind-body dualism.
[...] much of the debate over whether the cogito involves inference, or is instead a simple intuition (roughly, self-evident), is preempted by two observations. [...] As Descartes writes:
When someone says “I am thinking, therefore I am, or I exist,” he does not deduce existence from thought by means of a syllogism, but recognizes it as something self-evident by a simple intuition of the mind. (Replies 2, AT 7:140).