Most people book in advance for the visit to the Burj Khalifa which is cheaper than paying the immediate entry fee.
You can book a ticket online for 100 AED or pay the immediate entry admission of AED 400.
I would highly recommend paying the ticket online and booking it in advance because it's much cheaper although if you can't or don't want to book the ticket online you can still get a ticket when you show up.
Although you'll pay a higher price for the ticket to enter Burj Khalifa which is AED 400 VS the 100 AED ticket fee when paid for online.