Being basically lazy, and lousy at typing
I keep this link for anyone who asks this question...
http://turningpointpropellers.com/propellers_101_cm.htm
and this one...
http://www.propmd.com/t-propellerbasics.aspx#pit
and these...
http://unitedboaterscoop.com/html/propeller-basics-part1.html
http://unitedboaterscoop.com/html/propeller-basics-part2.html
http://unitedboaterscoop.com/html/propeller-basics-part3.html
and when you finish reading them,
you'll find they all say basically the same thing.
Install a tach, run with your normal load at full throttle,
buy the prop that lets you run in the top end of the power band.