Warn is a verb that is used to tell someone about danger!
Warn and worn
e.g. "I must warn those boys not to fly their kite near those pylons!"
The best way of remembering the difference is the first three letters: war.
War involves danger and getting into trouble. Warn is therefore all about danger.
Worn is the past tense of wear.
e.g. "Britney has worn a wig since shaving her head."