1) Study relentlessly. The only book you need is From the Ground Up. Read it. Do every practice exam you can find. There are practice exams on the Atlantic Region website. There are practice questions in the back of FTGU. There are whole books of practice questions that can be found that aren't cadet questionsl, but filled with multiple choice government speak questions that are good practice.
2) Know the world around you for the interview. Read the paper, read this site and know who's running the show at Parliament, DND, and your region.
3) Be active at the sqn in fund raising, optional activities and do things at high school. The don't want to give a scholarship to someone who'll just cut and run.
Good luck.