You can reserve online, but they do have a limited number of tickets they sell in person everyday. Last Ticket is sold @2:15 PM. There is BLM camping up the road about 10 mins.

Carlsbad Canyon Visitor center is the entry point to the caverns. There is an elevator or a walk-in to the Caverns. You must have a reservation, That's not the same day. You can do it online. It also has a gift shop, bookstore, restaurant, and bathrooms.