Use the Trace Into command to single-step through your
  program, entering each function or procedure as it is
  called. The shortcut key is F8.
 
  When you reach a call, Trace Into steps from the
  statement that calls the routine to the first executable
  statement in the procedure or function.
 
  Once you have verified that the routine works as
  designed, you can use Step Over to save time as you
  continue debugging.