When flirting with a girl, you most likely want to be flirtatious but not outrightly so, and this is the best way to go, as to build sexual tension and leave the young lady in more need of your company. One of the easy ways to achieve this is via text teasing.  Text teasing allows you to flirt as much as you want but still play it off so that things don't move faster than they have to. Work your feelings into every text conversation as naturally as possible, sticking to text messages that feel natural to you. If you're shy in person, you can opt for a more subtle approach, with an inside joke here and there.