This is a tricky one for those who get vowels mixed up; often people coming to English from a language that doesn’t mark vowels in the same way, such as Arabic, can get caught out by all our very similar words, especially when they sound almost the same.
To warn, a verb, means to alert someone about something which is about to happen, usually bad. You can issue a warning (the noun) or be warning (verb) someone about the problem.
Worn is the past tense of wear OR an adjective arising from it, and both words have two meanings: to have on the body, as in clothes (I will wear a hat today) or to do with erosion and damage through constant use or friction, etc. (the water of the river has worn through the rock to make a valley; I have an old, worn book that has been damaged by years of use).