Hello Phouwathaurr34
If you want to travel in the panoramic cars of the Bernina Express you need seat reservations:
https://tickets.rhb.ch/en/
But I think that these cars are already sold out for this day. I suggest to take other trains without mandatory seat reservation.
For this day, Saver Day Passes cost already CHF 119 (first class, full price), return point-to-point tickets CHF 114. You can also buy point-to-point tickets at the ticket office in Tirano.
Kins regards
Danhiel