first of all you have to register with irctc.co.in after registration you have to login then enter the stations, date, quota and click on find trains button. Train list will be displayed on right side of screen afterwards click on the button infront of train in which you want to travel. above that screen you will find sea position for perticular class for which you had clicked. click on book option infront of seats avalability and then enter your details and security number displayed below, now make payment (many options above). your ticket will get booked.
Regards