+1 on getting a third mag.
In addition to showing up a little early, be sure to identify yourself as a new shooter. IDPA folks are among the friendliest and most helpful in the shooting sports, love new folks and are enthusiastic about offering any advice, or pointers they can. All this really helps! Be sure you inform each RSO before toeing the line aswell. They will give you a little extra time and more thorough exlpanation of the course of fire. Not because they think youre stupid (LOL), but because they too once had first time butterflies.
You wont believe how much fun youre gonna have!