Answer:
I'm going to assume you're a guy.... Buy her flowers, chocolate, dinner. Dress up - wear a suit or tuxedo if the occasion calls for it. If you have a talent, use it - if you play guitar, play her favorite ballad. If you play the bagpipes, find something else to do instead. If you can make things out of wood, make her a box and put something special in it - love letters, jewelry, or something quirky. Like a super Nintendo game with all of the music changed to her favorite songs. Do something that shows that you will spend time and effort on her. Those things are far more important to give her than anything you can buy.
Be a good man: Let her hold your arm when you're walking somewhere, look into her eyes when she's saying something important, never, ever let them wander downwards. Drive sensibly. Plan ahead, but don't fear spontaneity. Be nice to her parents.
Tell her she's beautiful, and not just because you heard it from me. Tell her what you like about her: the way she laughs or smiles, the particular shade of her eyes, or the way she did her hair tonight. Avoid complimenting anything below the neck - it sounds creepy. Feel free to compliment her positive character traits like generosity and sympathy.
Be yourself (or at least the good parts). Be polite. Be genuine. When you say "I Love You", mean it. Mean it. Don't throw those words around, and don't play games with her heart. Treat her the way you want some young man to treat your daughter, and never ask for anything in return. Be confident most of the time, and vulnerable some of the time. Be honest, be punctual.
And when you're away from her, think fondly of her, reflect on her. Convince yourself that she's worth whatever you have to offer, and the rest will come naturally.