During the title screen, press A, B, C, A, B, C, B, C, B, and A on the controller. A buzz sound should trigger. Enter into the configuration screen, and from there change the mode to easy. Then press right on the d-pad four times for 99 ships.
To skip levels, do the exact same method as above, but once you start a new game, press A, B and C together anytime during a stage to skip to the next one.