The biggest issue, I think, is getting over the fear of rejection. It's going to happen. Everyone get's rejected. Some more than others. The only way to really get good at asking girls out is to practice and learn from your mistakes. If you've got an older brother, father, uncle or friend that is or was good at getting dates, ask them for advice. That's only going to take you so far though.
As you get over the fear of rejection, you'll become more confident and comfortable, and those are both qualities girls are almost universally attracted to.
Seriously. EVERYONE get's rejected. Even that dude on the football team that's tagged half the tail in your class has gotten rejected numerous times.