To send certified mail you would go to the post office and ask for a certified letter envelope.
Then when you place your mail into the certified envelope you'll pay the postage for the certified letter at the post office counter.
The post office clerk will then apply a postage sticker to the certified letter and not a stamp.
So no you do not need a stamp for the certified letter to be sent but you do need to pay for the extra postage for the certified letter.
The certified letter will cost you more to send than regular mail would.
That is because the person receiving the certified letter will need to sign for the letter to be delivered to them.