Surprised there wasn't more attention in the post edmastermind29 gave when they discuss buying a bowling ball.
There is a huge difference in bowling ball types.
If you pick up a ball that the lanes provide for you to use at the bowling alley... they are (covered with) plastic.
Those balls are not designed to hook much, and without extreme techniques, you won't be able to do anything to them.
Balls you purchase from the pro shop are generally covered with something that grips much more strongly. A reactive resin ball, drilled with a fingertip grip (and an asymmetric core)... is a pretty common setup you'll find when purchasing your own. They sell plastic balls as well, but really only for picking up spares (where you want it to be straighter).
What are the extreme techniques to hook a plastic ball?
The only way I know to get good hook on plastic (polyester) balls is to take the thumb out of the bowling ball (or hold it with no fingers in the ball whatsoever). Then, when you are just nearing the last part of the walkup, twist it violently sideways with your wrist to get it spinning [picture if your hand (palm up) were under someone's hand, and then you quickly tried to slap the top of their hand). If you can get that type of sideways twist on the ball right before throwing it, it will hook... but it's very hard to control because you don't have much grip on the ball.
With a reactive ball like you'd purchase at a store, it really is just about throwing the ball straight (bowl as if you are trying to shake hands with the pins), and then the ball does all the work.
I found a video about bowling ball types that is fairly interesting: http://www.youtube.com/watch?v=QDUl1512EaY
http://www.bowlingball.com/BowlVersity/bowling-ball-coverstock-classifications also goes into more details on the coverings used for bowling balls. Basically, if it isn't plastic, it should naturally hook some. It's not a skill as much as it is the equipment, unless you have someone bowling two-fingered (does occasionally happen, but no pros bowl that way that I know of).