Action script query

Can you please tell me how to debug action script by using step in command

or

var a:Number=10;
trace(a);
a=20;
trace(a);

Above is the 4 statement action script…I want to execute statement 1-2 and see the output as 10…then i want to halt and again by executing next statement i want to see value of a as 20;…

how to debug action code scrip STEP by step…i m a beginner …so pls explain in detail thanks