To run NetWorth, first you need Java 1.4 (or higher). You can go here to download a recent version of Java if you don't have one. (If you do, but are not sure of the version level, you can try running "java -version" in a terminal/command window and checking what it reports.) NetWorth is using a couple of the newer features only available in the latest "1.4" version of Java.
Obviously, you need a copy of NetWorth itself. Note that, currently, NetWorth is still in development (though useable enough for testing, evaluation, etc.; feedback welcome). You can get that here.
NetWorth makes use of the third-party JFreeChart library. If the bundle of NetWorth you downloaded above did not include JFreeChart (you would be told if it did), you can grab that here. (Note you will need to download the JCommon package there as well, as JFreeChart requires it.)
NOTE: Since this site is very new, this is a rather quick-n-dirty write-up.
Place the NetWorth, JFreeChart, and JCommon jar file packages somewhere convenient to you. (You can dump them all in the same directory, though you might consider treating JFreeChart and JCommon more like libraries; personally, I keep the latter two in my ~/lib directory to build/test against).
Set or modify your Java CLASSPATH environment variable to include all three jar files, then run "java org.networth.Main".