Descartes reasoned that he could not doubt that he’s thinking. Even if he doubted that he was thinking, his doubt was itself a sort of thinking. To doubt that one is thinking is internally inconsistent: The doubting itself shows that the doubt is wrong. Once Descartes knew he was thinking, it was a small step to knowing his own existence. Where there is thinking, there must be a thinker. So Descartes concludes: Sum! I exist!
