First of all, you need to meet one if you haven’t yet. You may try online using one of this 9 Best Dating Sites for Nerds in US. Or offline - special classes, video game tournament, Comic Con...
Always tell your nerdy girl that you like her and treat her a little bit better than you treat other girls. Showing her that she is special is the fastest way to get a girl's heart.
Make some really close friends. This is good because she will see that you can maintain a great relationship and she will see that you have friends to do all kinds of boyish things with them, so that she doesn’t have to drag around to watch a sport event which she is not interested in (but keep in mind that some girls love sports).
Smile and or wink at her to make her feel special.
Never tell a smart girl that her friends are stupid, ugly, taciturn or boring.
Be sincere: when you say something, keep it in mind.
Be educated and express yourself clearly - nerdy girls love smart guys.
Never use jokes with a sexual context. They are the main reason for discouraging interest, unless they are used very rarely and they are perfectly in place.
Witty banter, intellectual discussion... Definitely a good idea. As mentioned earlier, you have to make your nerdy girl feel like a heroine in a Jane Austen novel.
Read the literature (I recommend Pride and Prejudice, which has the simplest yet most romantic story of all time ".... Dear Beautiful Elizabeth" - Mr. Fitzwilliam Darcy has too many good lines to quote), this will help you come up with some good phrases (low-grade bait phrases will definitely not work).
