How can I get better at interviews?

I make eye contact, I have a firm handshake, I smile and am friendly to the person. It's a cinch for me before and after the question part. (I've only had 2 interviews in my life, I've been applying for about 2 months.) I felt comfortable with both, not even nervous. Just excited. But when it comes to the questionnaire, I draw blanks. It's like, a subconscious and out of my control feeling. I looked up all the questions I could that they might ask the day before, but I still wasn't ready, I guess. I know what I want to say, but the words won't come out in the order I think of them. I ended up getting jumbled, stumbled, and ultimately, apologizing for my behavior. I want to be outgoing, like I picture myself in my head being. Why can't it come out? How can I get better at it?