Be yourself/natural!
Sure read some books if you want, but really you need to shine and be yourself. I still think it's a good idea to prepare what your going to say. It doen't have to be much, but maybe write down what you want to say.
Also put yourself in the girls shoes, what do you honestly think she would like? Hopefully you the girl is similar to you and wants an honest and open guy with no bull.
Maybe even say, your were unsure if you should approach her, that you felt might come off us as a bit too confident. Most girls like honestly and even like to see that your not completly confident and that you have some fears.
You sound like a nice guy and it's nice that you're worried you're not good enough. Most girls have these insecurities too! In fact I would say everyone has these insecurities. I know I do
Go with your gut feelings