An important part of electronics is recognizing functional blocks that form larger circuits. This schematic is a drawing of a basic amplifier. R1 and R2 form a resistive voltage divider to set the gain of the amplifier. If you replace one of these with a potentiometer, you can adjust it. This circuit will work with just about any op-amp. EAGLE 4.16 format.

A few quick notes: You can use this to build a simple headphone amplifier. The greater the voltage you provide the amplifier with, the louder an output it can produce. Common op-amps include the TL072, LM386, LM741 and TL082. The goal is to have a bipolar supply. If you use two 9 volt batteries, you can connect them to get +9v and -9v. This allows the amplifier to go negative with it's output.