Link to the schematics please, but I'd say yes to the 340 ohm resistor (I'm guessing that it's for a current limiter for a LED)
get a AA battery holder and AA batteries
mAh is milliamps per hour
1mAh means you can use 1mA from the battery and the battery will last 1 hour
If you don't understand current, milliamps, or amps, it means you are still in school and should have a science teacher who can answer your questions.
when batteries are added in parallel, you add the mAh up, but when you add identical batteries in series, the voltage increases but the mAh stays the same.