KE = 1/2 M (mass or in this case weight of the projectile) x V (velocity or in this case speed of the projectile) ^2 (squared)
Momentum = MV (the velocity is not squared in this case nor is the resultant halved).
So in order to figure them you'll need a grain scale to weigh your arrow and a chronograph to measure their speed.
In this case your mass will be in grains and your velocity will be in fps so to get those units to lb. ft. of KE you use:
lb. ft. = (grains)(fps x fps)/450240
While I don't agree 100% with everything written in the article, I'm in agreement with most of it. Here's a handy calculator that let's you just plug in the numbers:
http://bowsite.com/bowsite/features/practical_bowhunter/penetration/index.cfm
Here's another related but much more scientific discussion of the two by a bunch of physicists:
http://www.physicsforums.com/archive/index.php/t-195119.html
(Pay particular attention to what mordechai9 has to say, I think he hits it square on the head.)