Take a look at the booking confirmation, printed or in email. The reservation number is usually displayed at the top.
6 digits from the booking confirmation "Reservation ID" (for example "134576")
6 capital letters (for example "CDZFVD")
letters "PC" and 8 digits (for example "PC52769591")
7 digits (for example "2602149")
3 capital letters and 6 digits (for example, "TCP-219423")
4 digits (for example "1987")