(2) I think
Therefore (3) I exist
The trouble with a syllogistic reading, which Descartes explicitly denies elsewhere in his writings, is that no reason is given why (1) should be immune from the doubt that the Meditator has posited. Also, the syllogistic reading interprets the cogito as a reasoned inference at a point in the Meditator's doubt when even reasoned inferences can be called into doubt.
But if everything is to be doubted, how can the Meditator know the cogito? A number of readings have been given to understand this step. One is to read it as an intuition rather than an inference, as something that comes all at once, in a flash. Another reading interprets the cogito as a performative utterance, where the utterance itself is what confirms its truth. That is, I could not say "I exist" if I did not exist or if I did not think, and so the act of saying it is what makes it true. Thus, I can only affirm my own existence (not anybody else's) and I can only do so in the present tense: I cannot say "I thought, therefore I was/am."
It should be noted that the cogito only works for thought. I cannot say, "I walk, therefore I am," since I can doubt I am walking. The reason I cannot doubt that I am thinking is that doubt itself is a form of thought.
After the cogito, the Meditator advances the claim that he is a thing that thinks, an argument called the sum res cogitans, after its Latin formulation. There are three controversies regarding the claim "I am...in the strict sense only a thing that thinks," which we will examine in turn: whether the claim is metaphysical or epistemological, what is meant by "thing," and what is meant by "thinking."