I am taking the example of irctc website. First of all register yourself to the website.
For example if you want to reserve seats of a train then select your starting location as well as your destination. Then it will ask you the date for your arrival, also select your class and the time matching your schedule . Then it will show you a list of trains according to the filter applied. Select the train from that list. Fill in the details like no. of passengers. Select the mode of payment like credit or debit card etc. Then it will ask your details for the payment of the tickets. Fill.in those details and click on make payment option. Your tickets will be booked or your seats will be reserved.