If you're training under commercial pilot standards or aiming for a commercial air transport licence (CPL, ATPL, or charter operations) then yes a Class 1 medical certificate is required. Regulatory bodies such as the DGCA (India) or FAA (United States) mandate a Class 1 medical for pilots who will exercise commercial pilot privileges carry passengers for hire or operate multi crew turbine-powered aircraft.
If you're beginning with private pilot training, many students start with a Class 2 medical certificate (in jurisdictions that distinguish them) or other less stringent medical requirements. In some countries the private pilot licence (PPL) may only require a Class 2 medical or equivalent. This is because private flying is considered lower risk, lower responsibility and usually does not involve compensation or multi crew operations.
That said, getting a Class 1 medical at the start of your training can be a wise move. Why? Because the medical standards for Class 1 are stricter more frequent renewals greater cardiovascular neurological, and vision testing and a higher fitness bar. If you begin training with a Class 2 medical and later discover you can’t meet the Class 1 medical standards you might find yourself unable to convert to a commercial licence or advance to a career pilot track without restarting or revising your medical eligibility.
Country specific regulation matters. For example the DGCA in India has detailed criteria and forms for Class 1 medical certification and the FAA in the U.S. uses its own standards. You’ll want to consult the guidelines for your aviation authority and possibly take a medical exam early in the training process even if your initial training doesn’t strictly require Class 1 getting the medical clearance early can avoid wasted effort if you later aim for a professional pilot career.
If your ultimate goal is commercial aviation jet flying or professional pilot work it’s highly advisable and eventually mandatory to have a Class 1 medical. If you’re only flying recreationally or privately you may begin training without it but getting it early can save time money and disappointment down the road.