measuring current from a photo diode

[mischl]

hello all

i would like to read in the brightness of light. i have a sun-wind-sensor with a build in photo diode (i think) in direction of barricade.
if i unterstod it right, the diode generate current in reference to the light. how can i measure this current (the light at the end) with my pic. i work with a 18f876a at port ra.0
in the attachment a jpg of my test, but this doesn't work allright. must i change this current to a voltage, how?

thanks for a hint