Like Cal bow said, arrow test kits is a good place to start if you dont know the spine you need to start with. You can also buy field point test kit. Once you find the correct size arrow,(correct spine). The point test kit comes in handy for fine tuning. I think it comes in wieghts from 100 grains to 250 grains. The more wieght you add up front makes your arrow act more weak.
Do a search for bareshaft testing. And checkout this website under bow tuning. Very good reference.
http://www.bowmaker.net/index2.htm For example, (assuming you are right handed) If you have an arrow that is too stiff,bareshaft hitting left of your fletched arrows, and you are shooting it with a 100grn head, just move up in tip wieght and it will make arrow shoot weaker, more to the right. Just keep moving up in weight untill you are were you need to be.
Also, cutting length off of an arrow will make it stiffer as well. For example, if you got two same size(spine) arrows, one cut at 28" will be stiffer than one cut at 30".
Good luck to you