My question for you is: Why do you feel ready? What do you want out of marriage that you aren't getting from your relationship now?
I'm 32 and never really felt like I was ready to (or needed to) get married -- because all of my relationship/partnership needs are being met through dating. But then, I tend to be a very independent person. At this point, I don't think I'll ever get married! But who knows... something or someone may come along and change my mind. ;)
it may feel like you're ready but you're so young. you haven't had much time to do things with your life! even at 18 you still probably haven't done a lot - just school and family. give it some time.