if this is a California case the California formula will apply. You must use one of the computer programs to calculate this; the formula is very complicated. It is based on net earning of Mom and Dad, the number of children and the percentage of time the child/ren is/are with each parent.
You can download a demo of "ex-spouse" and attempt this yourself but it's more then tricky.
The courts use dissomaster to calculate. You may want to check out the department of child support services (DCSS) website, which provides a similar calculator for free. A few years back these two calculators did not produce the same numbers but since last year they fixed the problem. The calculator is very straightforward. You need to input a bunch of information such as your salary, your time, etc., and you can play around with the numbers as well.
The other option is to go to the self-help center of your local court, where you can use the dissomaster on their computers for free.