Yes to visit Burj Khalifa you'll need to book the tickets in advance to visit the Burj Khalifa.
However you can only book the tickets to Burj Khalifa only 30 days in advance and no longer than 30 days so you'll need to book the tickets to Burj Khalifa 30 days or before in advance of visiting Burj Khalifa.
If you can't book the tickets to Burj Khalifa within 30 days in advance you'll need to wait until you can.
Actually I almost forgot I remember reading online before that you can get a ticket to visit Burj Khalifa when you show up but it will be more expensive that way.
So it's best to book the tickets in advance to save money as the cost of admission to Burj Khalifa online is 100 AED and the immediate entry admission is AED 400.