"Hello (name), my name is (name), and I've been interested in cello for some time. My piano teacher (name) recommended you for Cello lessons. I thought i would see if you were available to give lessons (when, what time, what days, where). If so you can e-mail me back, or call me (phone). Thanks! "
that's just a short idea of what I would say. Please rephrase to your preference. Hope I