I bought a girl I know from school & work a Valentines Day card. What should I write?

[Daniel]

I bought a girl from school & work a Valentines Day card. Here's the deal: I liked this girl for around 8 months, and she just found out about it from my sister, who also works with her, 1 day ago. I'm not really sure if she likes me, but I want her to know that I like her... a lot. We don't really talk too much due to the fact that I get /really/ shy around that I like. Don't get me wrong though, we do talk to each other whenever possible. The card reads "Whatever you're doing to be so wonderful, smart, and lovable... Keep it up." What else should I write, and how should I give it to her? She's in 12th grade, and I'm in 11th grade. Should I mention how I liked her for so long?