Go to a pro shop and have them watch you bowl
+1.
So many factors to list. Go to a pro shop with a good reputation that will take the time to watch you bowl a few frames. Then take their advice. More expensive doesn't always equate to best ball for your game.
If a shop tells you they do not need to watch you to sell you something, go to a different one. That indicates to me that shop will not be able to give you the best advice.
I would also recommend some lessons from a certified coach. The shop I go actually gives a free coaching session with the purchase of a new ball.
+100
Don't assume that the shop that is closest or most convenient is best for you. Of course if you're lucky, it will be. I drive an hour to get to the shop that drills for me and my son. And I do so without a second thought. If the shop is comfortable just selling and drilling without watching you bowl and getting your numbers, it is not the right place.
+100. I'm lucky to have a great shop only 20 minutes from me where my driller takes the time to watch me throw a few shots to understand what I want to do and get the PAP measurements. Make sure a pro shop will do this.
If the person just says "this ball will work" without watching you bowl, walk on out of there. That person just wants your money and doesn't care whether the ball will match up or not.
Also, if you give us a general location of where you are, many of us can recommend good shops for you to visit.