Hi,
You'll find lots of really good explanations for 'voltage divider' thru google.
I really rate Jeremy Blum's youtube tutorial on electrical engineering basics, and I keep going back to that one to make sure I've got my maths right:
There are also lots of Ohms Law calculators online which will sort out what values to use on your resistors, just be sure you cater for the maximum power your panels will be able to output, which as you say is a bit of an unknown for you right now.
Actually finding the voltage value in your Arduino sketch is pretty straightforward. Using AnalogRead() (analogRead() - Arduino Reference for details) you'll get a value on the pin with a range of 0 (corresponding to 0V in this case) to 1023 (corresponding to the maximum voltage your voltage divider circuit will provide, again the choice of resistors and the input voltage maximum will define what voltage that corresponds to. All values in between have a linear relationship to the input voltage, so to get the voltage, simply multiply by the maximum (say it's 4.6V as a result of your divider, just multiply the AnalogRead() value by 4.6) and you have your voltage that you can make decisions on in your sketch, fire off the relevant relay, light up LEDs, display on screen or whatever.
Hope this helps - sounds like a really fun project you're embarking on,
Geoff