Just to reiterate what has been stated, legs and core are where you will probably see the biggest return on your bowling game.  Stretching also will give huge benefits. 

But beware of one thing.  To gain ball speed, you will have to increase the speed of your feet during your approach.  However, faster feet can cause more strain on your knee when you have to bring this higher speed to a stop.  Make sure you are working on how to stop properly (knee continuation) to prevent any kind of injury.

If you want "bowling" fitness as compared to "general" fitness, there are some people who specialize in this, mainly Team USA bowlers and coaches (might be approachable for information).  If you want "general" fitness, I would consider getting with a trainer for a short period of time to get you on the right track with periodic review sessions.  Injuries are all too common with beginners getting into fitness and they usually lead to people quitting.  We don't want that.