Because it is not very easy to explain in few lines, it is complicated.
Just powering the 4 motors with same voltage/pwm doesn't means that it will lift up, it can flip and strike on the floor and will endup with damage. You need to use an IMU based control system to ensure the stability. Details regarding the battery, circuitry, algorithms etc will be there in the attached link. Please check it out.
My local hobby store sells regular model helicopters, quadcopters, airplanes and cars. All are radio controlled and most have accellerometers and gyroscopes built-in for good stability. All the flying models are powered from powerful but lightweight inexpensive Li-po rechargeable batteries designed in the USA. Cheap Chinese poor quality Li-Po batteries are available online from ebay and Hobbyking.
My local hobby store sells and I buy most products from the American company www.horizonhobby.com .