If you seriously love her, she may already know that you love her. There are so many ways to express your love. I love a girl so much and recently I expressed my love. How? I wrote all my feelings about her and how much I love her.
Write A Romantic Letter
I started writing the letter at midnight and completed it by the early morning (at this time all my feelings came out deeply). I feel that you can express your feelings more clearly by writing them, if you just try to tell someone, you often miss the crux of what you wanted to say. Then I came to know that she used to travel every day almost two hours journey by bus. She is a music lover. So, I bought her an iPod in a interesting color. Then, I kept my letter with the iPod in a gift pack and gifted her. While giving, I clearly said to her 'don't open this in front of anyone, open it when your alone'. I hope that this showed her I paid attention to what she says, and that I had really thought about what gift would be most useful, and make her think of me every time she uses it.
Just Go For It!
I feel that, from my experience, at least you may get some ideas. Just express your feelings to her, do not force her. You could treat her to a romantic dinner, even better if you find out her favorite and you cook it for her! You could tell her over dinner how you feel. After expressing your feelings, just leave her to digest what you've said. Don't ask her about her opinion on your feelings. If she also likes you, she might behave the same towards you. Otherwise, spend much time with her and take care of her.
I am sure that, if she feels comfortable with you and you have based your relationship on good foundations of friendship, that she will know even without expensive gifts or fancy words, that you are head over heels in love with her.