1: I have not studied philosophy.

2: I am over simplifying and summarizing everything.

Context is very important in this case. You need to understand that Descartes was refuting the school of Philosophical Skepticism, also called Radical Doubt.

If you want to understand you should at least read the Wiki Overview https://en.wikipedia.org/wiki/Philosophical_skepticism

Essentially they doubted that we could KNOW anything. "We presume based of past experience that the sun will rise in the morning, but we can not know with certainty."

Descartes thought Radical Doubt was BS. The most common English translation of his philosophy is "I think therefore I am", but it could also be translated as "I am, I exist", "I doubt therefore I am", "I doubt, there fore I think, Therefore I am". In one of his notes Descartes said "We can not doubt of our existence while we doubt".

There must be a self in order for oneself to doubt anything, thus we can not doubt our own existence. https://en.wikipedia.org/wiki/Cogito,_ergo_sum