The old general rule the proshop people use is bodyweight divided by 10 and round down - 155lbs = 15lb bowling ball should be about right. This doesn't apply to everyone so YMMV. Also, a bowling ball that does not get into a roll before contacting the pins will not be converting all of its energy toward the pins. 8, 12, 16lbs, it doesn't matter, though, as stated in the link by Electric Amish, a ball 14lbs or heavier will produce the best results. Optimum pin carry results from a ball rolling and entering the pocket from a 2 to an 8 degree angle. A straight bowler can obtain about a 2 degree angle by starting the ball from the left or right gutter without hooking it. It is possible to throw the ball as hard as physically possible and get strikes without getting the ball into a roll, but by doing so, you increase your chances of leaving the 5 pin (cough) or the split variations containing the 4,6,7 and/or 10 pins on head pin hits.
I throw my strike ball at 15 - 16mph with about 15 revolutions. My league shot is oiled (boards) 10 x 10 to 30ish feet and I always use a Columbia300 gold Quake (stacked leverage weight, cuz any other drills on this ball don't carry). I start the ball at 16, swing the whore to about 8 and back to 17, so 17 boards.....and I carry everything

Getting a cheap (Brunswick, Columbia, Storm, Track, ebonite<----eww)Urethane bowling ball drilled and fitted PROPERLY (NO K-Mart, WallyMart, etc...) will produce consistancy more quickly and allow you to troubleshoot your mistakes. Also, generous portions of cold beer are always necessary during any bowling session as it eases the pain of those sub-100 games.