Please refer to the project page for more information: http://code.google.com/p/zxing/ in particular: http://code.google.com/p/zxing/wiki/GettingStarted

To get started, you can try building and running the command-line client; you will need to have Apache's Ant tool installed to run this:

ant -f core/build.xml ant -f javase/build.xml java -cp javase/javase.jar:core/core.jar com.google.zxing.client.j2se.CommandLineRunner [URL]