It depends on her age and what kind of girl she is. If she appreciates you being straightforward then just let her know your feelings. In any case, its best to tell her in person so there won't be any confusion as to how you feel. Long letters can be ambiguous and little notes can get lost. But the look on your face as you confess to her won't leave any doubt in her mind =)
You could try confessing little things you like about her. Instead of just saying "I like you" it could be something like " I love the way you laugh" or " Its really cute how you ______" Whatever you say doesn't have to be poetic or super cheesy. As long as its honest and what you really feel, then you should go with it.