Where is the fuel stored on an aircraft’s wing?
This depends on the aircraft. In many cases there is a tank inside the wing. In some cases it can be stored in a soft bladder in the wing and in other cases the wing its self is the tank. 

Here is an in wing tank 


Here are some in wing fuel bladders. 


Mooney was known for having integrated fuel tanks in their wing designs. Here are some pictures of the tanks being resealed. They are notorious for needing a reseal every now and again but they do save in weight. Modern composite structures often don't lend to this design as jet fuel or avgas and composites don't really mix.   

  

Here is a nice cutaway of the A330 to show where the tanks are. 


It should be noted that you can generally pump fuel between the tanks however care must be taken when this is done because it can greatly effect the CG. This podcast has some really interesting information on how complex it was to manage the fuel weight on the Concorde and how it needed to be pumped around the tanks during various phases of the flight just to keep the plane stable.