In latest modification for Tatkal Ticket reservation rule, You can book Tatkal Ticket only before two days of journey, booking will start at 8:00 AM. You can book the ticket from internet ( http://irctc.co.in/ ) or Indian Railway ticket Counter.
Suppose your want to travel on 16th May on train on any particular train, and you have not got ticket reservation. You can get Tatkal quota ticket, you can book Tatkal quota ticket only 14th May 8:00 AM onwards.
Tatkal quota ticket booking starts on two day before the journey date. It start at 8 A.M on both Internet IRCTC or at counter.
Tatkal Quota seat booking from internet may not work all time because IRCTC website get overloaded at 8: 00 AM. You may find IRCTC website very slow at that time. On the other handat counter, if the person who book the ticket is working slowly, you may also find that at the time your turn comes, Tatkal Quota seat is full. Particularly for 2nd AC and 3rd AC, Tatkal Quota seat becomes full in around 5 minutes only. Sleeper class seat takes more time to become full in Tatkal quota.
So if you are at ticket counter, you need to be the 1st or 2nd in the queue.
If you are using internet (irctc), your broadband speed should be high.