Wrenne is spot on about the 0v and 12v.
As for the central position thing... you need to have the point blades in the central position when fitting the point motor underneath the board. You will need to wedge it or tape it as the spring in the point will make it spring to one side or the other. You also need to hold the bar on the point motor dead centre while doing this. Again, you can use tape or a piece of card.
About the switch... point motors are only designed to received a quick burst of power, if the power is left on for too long the motors coil will heat up and burn out. And this can happen in a matter of seconds! To stop this you can fit a CDU or capacitor discharge unit between the point motor and the switch. This unit will only deliver a very quick burst of power to the motor no matter how long you hold the switch in position.
Here's a link which explains all this is in great detail - http://www.brian-lambert.co.uk/Electrical.htm#Point Motor Wiring.