You have received some good advice above.
However, on the day of my intial interview (2 Feb 1955 just after 18.00 hrs) the first thing Group Captain Sammy Wroath the Commandant of ETPS said to me was “Why do you want to be a test pilot?”
I suggest that you won’t find the answer to that question in any book, but you still need to have a good one ready. I am pleased to add that after my reply the Commandant went on to ask more questions for 45 mins before bringing the interview to a close with the words “OK, you seem to be just the sort of young lad we need here but you will have to go away and learn to fly first”.
So I did. And eight years later I joined the course. In those days I was a civilian, with no flying training to my name, so I guess I would have been out on my ear after the first question had I not answered it to Sir's satisfaction.