Descartes was a geometrician. He found only in mathematics and geometry the certainty that he required. Therefore, he used the methods of geometry to think about the world ... Following his geometrical model, Descartes proceeds to doubt everything -- de omnibus dubitandum. He will suspend belief in the knowledge he learned from childhood, all those things "which I allowed myself in youth to be persuaded without having inquired into their truth." Doubt will be his method, a deliberate strategy for proceeding toward certainty. (Descartes is a doubter not by nature but by necessity. What he wants is secure understanding so he can stop doubting.)
Descartes finds that he has no trouble doubting the existence of real objects/events -- our senses too easily deceive us. And we can doubt the existence of a supernatural realm of reality -- figments and fantasies are too often conjured by our native imaginations. But now his geometrical model pays off: in trying to doubt everything, he discovers something he can't doubt. What he can't doubt is that he is doubting. Obviously, I exist if I doubt I exist. My doubt that I exist proves that I exist, for I have to exist to be able to doubt. Therefore I can't doubt that I exist. Hence, there is at least one fact in the universe that is beyond doubt. "I am, I exist, is necessarily true each time that I pronounce it, ot that I mentally conceive it."
Descartes thus becomes the author of the most famous phrase in Western philosophy: Cognito ergo sum, or, in his original french, Je pense, donc je suis -- "I think, therefore I exist."