At age 18 you can date whoever you want, however it is a different case if you are living with other people, especially parents. Whoever owns the house, or pays full rent, etc makes the rules. Don't like it, move. Broke? Maybe you should go to college and get more money so you can have your own place, love. And focus more on education and your own place then dating anyway.
And maybe you should consider why your dad doesn't want you to date the guy. Your dad's probably not your enemy, he's just helping you. Why hurt someone who loves you? Weigh in all the perspectives and make the choice that will best help you.