In very much the same way you show a gentleman you're into him: eye contact, smiling, unnecessary touching of arms or hands, showing interest in the things that interest her, and ultimately asking her out -- or saying yes if she asks you. Give her flowers or some other token of affection, like a book she mentioned or a food she said she loved. Mostly all the same rules apply, regardless of the gender of your object of affection.
You face a potential barrier in dating women if you never have before, because there's caution in the lesbian community about dilettante women who are just experimenting, playing around, or otherwise not serious about romantic connections with women. No one wants to get hurt in a relationship, and there's a fear that women who are "trying it out" might just be using the other woman. Justified or not, it's a real fear.
That means your approach should include both honesty about your inexperience and assurance that your attraction to her is genuine and not just some fling or experiment.
I kind of wish your question read, "There's this woman I really like, but I've never dated a girl before and I'm nervous about making the first move." When that is what's going on in your mind, that's when to start the flirting.
