I would look at the Amps hook up manual. Look at the ohms for output, check your speaker ohms.
The basic answer is this. Lets say your Amp can handle from 2 to 8 ohm speaker setups.
Now someone help me if I screw up the math.
If you have 2 8ohm speakers and hook them in series (assuming the same wattage speakers) You will have 16 ohms on the speakers...outside the range for the amp.
Lets say you hook them up in parallel...(both speaker + to the + on the amp, both speaker - to the - on the amp)
now some math 1/((1/8)+(1/8))=1/(.125+.125)=1/.25=4
So with 8 ohm speakers in parallel, you are at 4 ohms...That matches the amp.
Now if your speakers are 2 ohm, then you cannot run them in parallel, but mathmatically you could run them in series...I don't know if that is a good idea or not.
For the rest of your questions...it depends. But the cables you want the shortest are the line level outputs from the computer to the amp/preamp. The speaker wires can be very long.