How can I calculate the fuel required for a specific route and aircraft?
If you just want something simple, just look up the average fuel consumption for the aircraft type (in gallons per hour), and multiply that by the duration of the flight. Although airlines have more sophisticated tools to plan refuelling, this is how GA pilots estimate how much fuel there should be in the tank.

Official fuel consumption figures can be hard to get hold of because the Pilots' Operating Handbook and other documentation for the aircraft might not be publicly available, but it's easy to find out rough figures from web forums populated by pilots of that type.

If you are interested in something more complex (and accurate), see How can I calculate the fuel consumption of an Airbus 320-200 at various loads?