How do you book Ahmedabad to Goa bus tickets?

You can book Indian Railways tickets through Cleartrip banners in the website EquityCraft.com. Click on Trains. Select "Delhi All Stations" to "Goa All Stations"; select departure date and Class of travel and click "Search trains".
Three trains are displayed. You can book whichever is convenient to you.
The fastest train is Goa Smprk K Exp (2450) from H Nizamuddin to Madgaon with 8 stops and taking 29hrs 50min.