Measuring output of solar panels for science fair project
Unfortunately, it's not that easy.

If you connect different loads to a cell, the voltage and current through the load will (of course) be different. If you draw a curve of all that current vs. voltage points, it will look like the violet one on this picture:



If you measure the voltage with a multimeter (no other loads connected), no current is flowing through it, and you get Vmax.
If you measure the current only, you build a short circuit, so there is no voltage. This way, you measure Imax.

Now, just multiplying Vmax and Imax gives the light blue rectangle (partially overlapped by the yellow one). But this is no correct value because as said, the current at Vmax is zero, and so is the power!

If you take your measurements with the loads and draw a curve of (voltage * current) vs. voltage, you get the green curve. It shows what power is delivered by the cell at what voltage. You can clearly see there is a maximum, and the according power is the yellow rectangle.

So, the yellow rectangle shows the real maximum power, which is much smaller than what you calculate by Vmax*Imax.

I think you can measure Vmax and Imax for all your configurations of the cells, and it gives sort of hint which one gives the most power. But it may also be interesting to apply several different resistors and to this voltage and current measurement.



EDIT:

For your project, you could use a potentiometer, which looks like this:


By turning the knob, you can change the resistance between one of the outer and the middle terminal. It would be best to have two multimeters, one to measure voltages and one to measure currents. Connect it like this, leaving one of the outer terminals unconnected:







simulate this circuit &ndash; Schematic created using CircuitLab

and you can easily turn the knob a little, read current an voltage, and enter it into an excel sheet to generate curves similar to mine. 

The potentiometer must be able to handle the maximum power your cells may deliver, which can be estimated by Imax * Vmax (per cell). I'm also not sure what resistance to recommend, may be you use two, one in the range of 100kOhm to 1MOhm, an one in the range of 1-10kOhm, to be able to measure with low and high loads. May be, you have someone to ask for this stuff, like a teacher?

Further thinks you could measure (with one cell):


Various light levels
Cell in direct sunlight outside and direct sunlight through a window
Hot and cold cell


As said in the other answer: It's really nice to hear that you are interested in this stuff at that age, and I really encourage you to keep on.
May be, you can show us your results later on?