What is nine eleven? It's been causing me a lot of trouble not knowing?

Hey, I really don't know much and I don't watch the news much and I want to know what exactly is nine eleven. I know it has something to do with the world trade center and the twin towers. My siblings have been causing me a lot of grief and have been laughing at me and making fun of me because I didn't know what 911 was. Because of all that it caused me to go out of control and attack one of my sisters who was laughing at hysterically. And she told me I'm stupid, I'll never get into honors cles, I'll never get into a good college and I'll be a garbage man when I grow up. And because of that my dad came in and saw me punching my sister and he didn't even know what happened so he just took it out on me.