If you want to really kick your performance of the song into high gear, you've got to strap on a harmonica. And you can't just play any song on any old harmonica. Make sure you get a harmonica in the key of C or it'll sound off. In general, most starter harps that you get will be in this key, so play along to a song you know is in C and see if it sounds right to check whether or not you've got the right kind of harp. Marine Band harmonicas can be upward of $30, but are durable and have a high-quality sound, while other cheaper started harps can be much less. Following in the footsteps of Joel, Neil Young, and Bob Dylan, put your harmonica in a harp rack around your neck to free-up your hands to play piano and harmonica at the same time to complete the song. Typically, harp racks are available at guitar stores and other music stores and only cost a few bucks. They're cool tools to have on hand to add a little harmonica color to your songs. . Purse your lips together as if you were going to whistle, and place them on the center-most hole of the harmonica's keyholes, which should be the fifth from the left. By blowing through (exhaling) this hole only, you will create the note "E." Experiment a bit to learn how to make different sounds on the harmonica. By inhaling through this or any keyhole, you will create a sound one note higher than the blown note. The notes follow standard piano formation, meaning the blown notes to the right of the E are, in order; G, C, E, G, and C, while the inhaled notes are F, A, B, D, F, and A. Billy Joel would be the first one to tell you that it doesn't take a rocket scientist to play the harmonica part. Since the harp's in C, you won't be able to really blow a bad note, so it's mostly about experimenting with blowing and inhaling in the proper positions to get the melody close. Basically, you'll play E, G, E, C, alternating sucking in and blowing out. Listen to the song and you'll be able to get it after a few tries.

Summary:
Get a C harp. Get a harp rack. Put your lips to the harmonica correctly Play the melody during the harmonica break.