I am not new to hunting, but I am pretty new to bowhunting. For everything I have been reading, nothing tells me how to pick the grain of your broadhead. I am shooting at 45 lbs, have a draw length of 25 1/2" and shoot blackhawk vapor arrows with 100 grain NAP crossfires. My local pro-shop set me up and I am really satisfied. I shoot nice groupings,I even got a turkey this year with my bow, but what exactly tells you what grain broadhead to use, or even what weight arrow for that matter? I would really appreciate any feedback. I trust my local pro-shop, but I am the kind of person that wants to know why? Thanks!

As much as I hate to quote Hunters Friend,

http://www.huntersfriend.com/arrowhelp/arrow-selection-4.htm

about half way down the page, they talk about front of center. That should give you an idea where to start. BTW, they say 7-15%... Make it closer to the 15%

Zac gave you some great info and I would also probably use FOC as a guideline for now. Typically though, most of the average adult shooters I encounter are shooting 100 grain points on their shafts. Ofcourse, that depends on their individual setups but as I said "average".