Don't give up! Personally I think it is an excellent project idea.
avoman gives sound advice. Plus, a couple of places worth checking are
this site for specific information on the 737 and the section on fuel in
this page.
The second link does relate primarily to private flying, but the principles are similar, it is just scaled up somewhat.
One thing I would bear in mind, check the requirements of your assignment carefully before you go into too much detail. Your examiner/tutor is probably going to be more interested in the principles and methods than the minutae of precicely how many kilos of fuel are burnt per passenger.
Above all, good luck.
