My advice is just to kind of slip into it, going down on a girl is very nice I actually enjoy it a lot more than receiving (then again I am sexually extroverted).

Start with indirect stimulation to the clit (around it, not right on it) and slowly lick around. Just enjoy the taste, pretty much everything you can do will feel nice to her as long as you are gentle. When she's wet enough you can slip a finger in (remember the G-spot is about 1.5 inches in on the upper wall). You can go for direct clitoral stimulation but pay attention to the noises she likes, some girls don't like that.

Just enjoy it, its pleasing your lady friend is fun for both partners!