Go get a boombox and a cassette tape of "In Your Eyes"- Peter Gabriel and stand outside a girls window holding the boombox over your head. Works like a charm and you don't need an instrument.
YES! hahaha
I agree with a few posts in here. I'd wait until the relationship you're in is somewhat serious. Personally, I find guys doing things like that romantic...which might be better later in the relationship, not right from the get-go.