I WANT TO CLARIFY DOUBTS. ACTUALLY AS A TESTER WE HAVE TO FIND THE BUG AS EARLY AS POSSIBLE, ENSURE THAT THE BUG HAS BEEN FIXED.BUT IF SOMEBODY ASKS HOW EARLY?MEANS WHAT SHOULD BE THE CORRECT ANSWER.
First thing I will recommend to you. Turn caps lock off. You are essentially shouting.
Second thing I will recommend. Do a search of this forum and you may find your question has already been answered. It sounds like you are asking about unit testing, so if you search for "unit testing", I think you will have enough reading material to keep you entertained for quite a while.
As close as possible to the point at which the bug was introduced in the first place.
"... and now ... ow ... ow ... from the great state of Minnesota ... ota ... ota ... ota ... our favorite son and darkhorse candidate ... ate ... ate ... ate ... seeking the championship belt for the ninety-seventh time ... ime ... ime ... Put your hands together for Leeeeeeeessssssstttttttteeeeeeerrrrr Luuuuuuuutttttthhhhheeeerrrrr!" <- right about here should do