Is it a case of a girl telling a girl or a guy telling a girl? I will assume the former. What is it about her that makes her special? Think of it. Make a note of the reason. Why do you have these feelings around her? Why HER? What makes her special? Is she the One? Does she seem like she will feel the same way for you? Is she okay with this? If you think she is, then tell her- you could do it slowly gently, or tell her out straight. Pick a right time and place. Does she give the vibe that she is into you also? If so, it will be a wee bit easier then for you. Is she your type? Are you her type? Are you guys a match? To be love - you both must know each other pretty well already. She must know what kind of person you are.
Just be very honest. Look in her eyes, that will tell you. Has she hinted to friends you both know? You could give the hint through a friend back to her. There's lots of ways to tell her. What does she like? what kind of things? Get her something she will definitely like, and pick the moment. Cards, chocolates, gifts, flowers - pendants, - you name it. Pick you gift. And go for it!
Feel the fear and do it anyway!