Noob with LED drop detector issues

Hey guys, I'm still pretty new to programming and coding and I'm struggling with this project that was given to me. I'm suppose to create an LED motion detector that will detect and count the number of drops of a solution added to another solution. I have an arduino uno and 1 pair of led infrared emitter and detector (#276-0142) from radio shack and the resistors etc. Am I missing any major components and how do I go about getting this project underway? Thank you for any help. I'm just really struggling at trying to figure this out.