There are so many different ways to express your love:
Look directly into her eyes and sincerely tell her how you feel about her.
Write her a letter or poem expressing your love for her.
Show her through actions: a gentle smile, holding her hand, a kiss, a display of chivalry (as in opening the door for her), or doing a task for her without being told.
A simple gift, such as a bracelet or flowers, may also be a nice touch.
All the best.