I assume you are a foreign tourist. In India, you cannot just show up at the railway station, and expect to get seats. There is always a high demand for trains , which are the country's lifeline, and usually run 100% packed. My recommendation is that you should make all prior plans, class of travel, seating, source and destination stations - get a reservation for everything.
Indian train fares are among the most affordable in the world. Availability of seats is the main problem.
Good luck, welcome to India, and happy travels.