The best preparation for taking a CELTA is to work for a couple of years. The course is, in my opinion, wasted if you do not have some experience you can relate to what you are learning. Without experience you will spend most of the time trying to work out the basics and the CELTA material will go over your head.

So, I'd suggest getting an entry level job, working for a year or so, and then returning to a CELTA to move higher in the industry.