First of all, you don't have to make a seat reservation, i.e. it is not mandatory. But with a first class ticket, seat reservation is included, so you'll get it.
During the booking, you can choose the compartment type (open saloon car or compartment), window or aisle seat and quiet zone/phone zone. if you proceed to the next step/page, you'll see a summary of your seat request and there is a link to 'Select Seat' which offers you to pick a seat, just like you would pick a seat on an airplane. If you don't go for that option, you will be allocated a random seat, and car numer and seat number will be shown on your ticket.