Spending Credits
Credits are used to play on tables at the cost of one for each play, but you can also buy tables for 100 credits. This allows you to play the table for free as much as you want.
Earning Credits: Basics
Initially, the game gives you 20 credits to spend and four tables to play on. You can earn more credits in the Tournament and Williams Challenge modes, but not in the Practice Arcade mode. The most basic and simplest way to earn credits in these modes is to just clear a table by attaining the highscore. Beating the previous highscore on a table can earn you 20 credits.
Earning Credits: Advanced
In addition to simply beating the highscore of tables, you can also try to complete table goals for five to 10 credits each. Table goals could be anything from earning a 10,000-point bonus to getting the Rudy Gulp bonus (on the Funhouse table). Table goals can be very challenging and real tests of your skills, but completing them all nets you a tidy sum of credits, as well as earning you some nice unlockable rewards.
Earning Credits: Expert
The final method for earning credits is similar to the last but boasts a significant increase in difficulty. After completing all the regular table goals, you have the chance to take on the Wizard Goals. Each Wizard Goal is worth exactly 10 credits and completing them all will unlock new options for the table. These goals can range from simple tasks like scoring 4,500,000 points on a table to difficult objectives like relocking both balls in the Express Lanes during multiball (on the Taxi table).