Tatkal opens one day before the journey date...
If you are seeing 11th it will not be possible for you to book today.
and 18th is out of question...
1-Please specify the 'from' and 'to' journey stn...
2-as...
more... well as Class you wish to travel...
You can see at this link :
click here the 'available quota for tatkal for above train for 2AC and 3AC...
and here
click here Sleep Class