It honestly depends on who this girl truly is to you. If she wants a simple "Happy birthday!" that's fine. If she is your best friend, you can write something like "Happy birthday! You are truly an amazing friend that I never want to lose. I hope you have the amazing birthday you deserve!"
As for signing the card, try to write something less formal and more fun. Try to avoid a bland "from, (your name)." Try "your friend," "sincerely," or "love."
Hope this helps--if you want any more advice I'll be happy to help.