You really ought to be doing your own homework, young person.
Descartes was out to solve a problem. Like most everyone living in Europe in the 17th century, Descartes was a believer in the christian God story. That story included notions about the existence and character of Satan. Satan was understood to be a very tricky fellow who could fool anyone's mind into believing all sorts of things. Descartes was trying to figure out if there was any idea he (or another) might hold which was outside of Satan's ability to deceive.
He reasoned that there was only one single idea that met this requirement - Satan could not trick a conscious being into believing it existed if it did not.
"I think, therefore I am".
Pretty ******* smart, I think. It's one of my favorite ideas ever.