I was accepted into the ab initio program and I will direct my response this way. You didn't specify, but I will assume (yikes)!
The essay should be very sincere telling them why you want to be a pilot and undestanding that it will take lots of hard work and dedication. Just make it a well thought essay that shows your desire to become a great pilot, thats it! At the interview make sure you tell them you have the necessary flight funds to cover the program even if you are still working things out (loans,etc). They don't like to admit people that may come up short with their flight account. Have fun, it will be a great program!