Paella is Spanish, and in Spanish ll sounds like y, so the middle of the word is yel. American dictionaries show pah-YEL-uh. British dictionaries show pie-EL-uh, with an eye sound at the start. Both put the stress on the second syllable.
The word came into Spanish from Catalan paella, from Old French paelle, which came from Latin patella, a small pan. So paella is named for the pan it is cooked in. The same Latin word gave English patella, the kneecap.
The first syllable is the one that changes. Say pah or pie, then land on EL, and end in a light uh. Do not say pay-EL-uh.