To ruin means to wreck or destroy. If you're wearing a long dress, make sure your clumsy brother stays away from you. If he steps on it, it'll rip and be ruined.
Bad news always ruins your day and a guest's bad behavior can really ruin a party and all your fun! Be careful when adding salt to your food — too much will ruin it. Often you will see ruin used as a noun that means the state of ruin, the action that causes it or the destruction itself. If you are a ruin of your former self, you're in pretty bad shape. Use ruins for the remains of old castles and Roman structures.