First, when rubbing her clit, up-and-down motions usually work better than side-to-side or circular. Sure, you can use the latter two to vary it every now and then, but, on the whole (whether fingering or using your tongue), up and down on the clitoris usually works best.
When actually using a finger or two inside of her (I'd suggest one for tighter girls--you CAN fit two, usually, but you lose a lot of maneuverability, and she'll be sore afterward), one of the better motions I've found is to push your finger all the way in, then bend it slightly when pulling it out, so it scrapes against the top of her insides. Not only does this feel really good, but it gives you a chance at hitting the G-spot. If you do hit it, you should be able to tell by your partner's reaction. And, trust me, you need to find the G-spot sooner or later (sooner the better), and use it to your full advantage. That's one of the best places to start consistently rubbing when she's getting close to orgasm.
That's another thing- try to be consistent. You can vary your angle and fingering style sometimes during the session, but don't change every three seconds. And when she gets close to orgasm, DON'T change unless she tells you to. Get a little rougher and pump your finger a little further into her, great. Go faster, wonderful. But DON'T decide "Hey, maybe if I completely change my angle and rotate my finger THIS way" when she's near the edge- every time you switch, it sort of knocks her orgasm back a bit, and that can be really frustrating for her. And most importantly, when she's getting closer to orgasm, if she says for you to do something, DO IT. Harder, faster, rougher, whatever- she knows what she needs, and it'll help her orgasm along a lot if you just listen to her and do what she says. This is for HER, after all.
Also, pay attention to more than her genital area. I know it takes a lot of concentration to finger a girl, especially when you're new at it, and so you sort of forget to do anything else, but it makes the girl feel better and increases her general body sensitivity if you nip at her thighs, rub her stomach or breast with your free hand, do SOMETHING. Mostly, it helps her feel like something more than just genitals to you, and that means a lot to her general pleasure. You may even want to stop every now and then (provided she isn't close to orgasm because that would be just mean) to just hug and kiss her for a minute or two. It would give you both a break and would preserve the intimacy of it all.