basically you have a train an hour - booking is better online but if you are over 60, you can get a tarjeta dorada to get extra discount.
you can also go to the train station in Alicante itself, but why bother if you can do it online?
you get via email your ticket, with assigned seat (boy, can they learn - or teach - something to airlines) and that is about it.
if you are 4, there are extra discounts and you can book a table to be between you.
Seats in 2nd class are very comfortable. And of course you have a very large cafeteria / bar on the train.