Do you have an irctc.co.in account? If yes, just log in and go to Book Ticket, and fill in the necessary particulars and follow the instructions.
If you do not have an irtc.co.in, create one by going to the site and create an id. After some time you will get confirmation of your id by email. Use that link, and then proceed as above in the first para !
Good luck and good wishes.