I like a guy and I need help please?

Well I like this guy, he sometimes gives me attention, we never actually went out but we talk alot on bbm and msn, but most of the time I'm the one who starts? We say Hi to eachother everytime I see him somewhere, But the problem is , this guy is really complicated, sometimes he ignores me when we talk and sometimes he gives me complete attention?Well he told one of my friend that I'm like his sister, but do you think that can be changed?What Should I do to make him like me back and ask me out?