I'm fairly new to the sport as well but as far as I know you need to find the best kinetic energy your set up will provide. This would be accomplished with a combination of arrow weigh and speed. A lighter arrow requires more speed to reach the same amount of energy as a heavier arrow at a slower speed. Not knowing what set up you have it would be hard to say what the best way to go is (Lighter faster or slower heavier). Someone with a lot more experience will probably be able to help more than I can.