Correct spelling, explanation: hilarious comes from Greek hilaros, which then later gained the suffix -ous to make it a proper English adjective. Besides this little change, the word is basically unchanged; there was no need to add or get rid of, or exchange vowels. This is why you just need to remember that hilarious is written with one l.

Definition of hilarious:
adjective
1. extremely amusing, funny
I love watching cat videos on Youtube, they are just hilarious!
It’s hilarious that he still tried to convince his wife he didn’t cheat, even when she caught him red-handed.

Incorrect spelling, explanation: you may be tempted to write hillarious if you think the word has its roots in the word hill, or maybe the name Hillary, but both these theories, and the word itself, are incorrect. Hilarious comes from the Greek hilaros and this explains its current spelling. The English added a suffix, sure, but the structure remains as it was, so just remember that hilarious is far from hills.