Photodiode + OpAmp + Arduino: Further Amplification
What you are building is a transimpedance amplifier. They get pretty tricky pretty quick if you are not careful. I would look in to the OPT101 if you are able, it can make things pretty simple for low sensitivity projects. But to directly answer your question, you need more gain. A transimpedance amplifier works by converting a current (the signal that comes from the photodiode when a photon knocks some electrons loose) into a voltage through the feedback resistor. The output voltage at DC is really just V = I * R, where I is your photocurrent and R is Rf or your feedback resistor. Need more gain? make Rf bigger. There are quite a few problems with this in practice, but you should give it a shot first and see if you have any issues. Give this a read if you can as well as this book if you are interested further.