You can store energy in a battery or fuel (chemical energy in bonds), water behind a dam (gravitational energy), in moving fluids like wind or water (kinetic energy), or in the energy between sub-atomic particles (nuclear energy).
So, not enough information for me to help with your coursework.
Yep. agree with the others. What is the system? If its a spring i depends on on the material (the force constant for the material that the spring is made from) and if its is elastic or inelastic and the amount by which you have extended your spring (the displacement) – Hooke’s Law. But that is just for a spring. If its for a reservoir water turbine, it depends how high your mountain is and how big your reservoir is on the top of it.