Mine is a Lumix FZ35, not sure what specific model you have, but this is what to do. Hopefully this helps.
Set the mode dial to "A" (aperture priority). Adjust your LCD display (cycle through the modes by pressing the Display button) until you can see Example: "F4.0". That is your aperture size. Moving the joystick to the left or the right switches you between exposure adjustment and aperture. Make sure you move the joystick to the right to select the aperture adjustment, and then by moving it up and down you are able to adjust the aperture size.