Pay for work beyond 8 hours a day or 40 hours a week. The law says you must be paid at least 25% more.
On your payslip, as 残業手当 or 時間外手当.
Write down your hours every day. Unpaid overtime can be claimed later, and the 労働基準監督署 accepts complaints.
General explanation, for guidance. Rules and amounts change: confirm with the responsible office.