Where do cells get the energy they need to create the energy-rich compound ATP?

There are lots of different kinds of cells that exist; therefore, describing a single method by which every cell makes ATP is not possible.  I believe that the question is likely asking about human cells.  With that in mind, the general, broad answer is that cells create ATP energy from chemical reactions that begin with glucose and oxygen. 

The process is called cellular respiration. The basic formula is as follows:

Glucose + Oxygen --> Carbon...

There are lots of different kinds of cells that exist; therefore, describing a single method by which every cell makes ATP is not possible.  I believe that the question is likely asking about human cells.  With that in mind, the general, broad answer is that cells create ATP energy from chemical reactions that begin with glucose and oxygen. 

The process is called cellular respiration. The basic formula is as follows:

Glucose + Oxygen --> Carbon Dioxide + Water + ATP

Cellular respiration can be divided into a few main steps.  The first step is glycolysis.  It will produce a net gain of 2 ATP.  The 2nd step is respiration.  This step is also called the Kreb's Cycle.  It will produce 2 more ATP.  This is followed by the electron transport chain.  In general, it will produce a 34 ATP units, but that number can vary depending on the cell type.  That gives a total of 38 units of ATP that cells produce from glucose and oxygen through the process of cellular respiration.