When does a note become a flat or sharp on a sheet of music?

[Charles]

I have to take a chorus exam tomorrow, and I can't find out when a note becomes a sharp or flat. I've been doing the games on musictheory . net, but I've been guessing on them. How many sharp or flat symbols does it take to make a note sharp or flat? (If I sound like an idiot, it's because I know nothing about music theory)