ask (repeatedly), and i'll probably get around to answering.

"how do you say 'stupid' in spanish?"

"tonto," if the person, object, or concept to which you are referring is masculine. "tonta," if the person, object, or concept to which you are referring is feminine.

"estupido" and "estupida," while tempting quasi-cognates, are far stronger and more offensive than "stupid," and should be avoided by beginners at insulting things in spanish. don't rush into it; you've got your whole life and a fantastic array of profanity and insults ahead of you.