To get the best scores on "3D Pinball: Space Cadet," you should complete missions, keep your ship fueled, perform reflex shots and enter the red wormhole. Completing mission objectives increases your rank and allows you to get the most points in the game.
Your current mission objectives are listed in the lower right corner of the screen. Refuel your ship by hitting the targets in the upper left corner. Nudging the table correctly can be a lifesaver in tough situations.
If you want to have an extra boost, you can find a list of cheat codes on GameFAQs.com; simply enter the sequence of letters in-game before launching your first ball to unlock cheats such as unlimited pinballs and higher rank.