Try different things and ask your girlfriend to rate it, from 1 (awful) to 10 (excellent), so you’ll know what she likes and dislikes:
Try the following strokes on her clit: lazy circles around the clitoris, short rapid up and down strokes, side to side licks immediately underneath the clitoris or on the clitoral hood.
Try the ‘ice cream lick’: keep your tongue soft and wide and lick her vulva from the inner lips to the clitoris.
Try sucking her clitoris. Put your lips around her clitoris and suck it in softly. Then you can keep it in for a few seconds. You can also use your tongue, pushing her clit in and out.
Suck her clitoris and start humming. This light vibration can be a turn on.
Try both direct and indirect stimulation of the clitoris.