To start debugging your program first compile it with "javac -g MyProg.java". Notice the "-g" flag. This causes the compiler to insert debugging information. Then type
jdb MyProg stop in MyProg:main run listThis should show you the first few lines of your program, with an arrow pointing at the first executable statement in main(). Type help to get a complete list of debugger commands. Here are a few.
For more information view Sun's documentation for jdb.