The girl should like you... at least a little before she can get any sort of butterflies.
Compliment her or say something nice to her. Do cute little things. Its actually quite easy. I get butterflies just reading this question.
1. Stare at her in the eyes and smile and she'll ask "what" and you tell her that you love her eyes. 2. Rest your head on her shoulder 3. tickle her! 4. wink at her 5. hold her hand 6. Buy her something thoughtful even if its something small 7. stand up for her 8. take her side 9. give her a hug