Hello RenWang
Thank you for your message.
You can make seat reservations online for IC trains (if a reservation is possible) online. For ICE trains, please contact a ticket counter directly.
In Switzerland, you can board and travel on most trains without seat reservation (in principle, only on tourist trains is seat reservation obligatory). However, it may then be that there are no more free seats (no seat guarantee).
I wish you a good day.
Kind regards,
FatimaH