I am making a USB charger, that you can plug into the wall (so i don't have to run my computer to charge my ipod) i have a DC adapter plug that gives me 9V and 800mA, so i am using a 7805 to give me my 5 volts but how do i get the 800mA to be about 500mA? Any thoughts..

You don't. Whatever you plug in will only draw what it needs. You need to make sure the voltage is matched (which you did with the 7805) but the current will be fine so long as your power adapter is equal to or greater than what is required.

Actually, having a little over 500mA available is a good thing, iirc some devices (like iPods) use a tad over 500mA when charging.

V = IR. Set any two and solve for one.

If you already chose a V, and you cannot change your load (R), then you cannot change what I will be either.