Give it some noise by hitting your phone to get it going.
Change the pot to change the frequency. The capacitor can be changed as well, but it is usually easier to change resistance in applications like a stylophone. I got this circuit out of "Engineers Mini-Notebook: Op Amp IC Circuits"
Blue is the output, green is the non-inverting input (reference voltage), and orange is the inverting input (capacitor voltage).
In this example, the feedback resistors are both equal to 100k, making the reference voltage half of the output voltage. That means the frequency will be 1/(2ln(3)RC), or about 1/(2.2RC). R is the pot and C is the cap.
