Hold 1 + 2 + Up and power on the Sega Master System. The music will keep playing after the Disney logo disappears. Rotate the D-Pad clockwise until a chime is heard. Select the "Start" option at the title screen to choose a new starting level.
You can get thousands of points by waiting until the last few seconds of the level before completing it. Make sure that you have done everything else before waiting at the end. Then, when the counter is counting up your points, it will keep counting for quite awhile.