There are many tools used for mesh modeling that you can use, the simplest to get you started is probably the knife tool.
In edit mode press K and then click where you want to corners of the new face (also called a polygon) then press ⏎ Enter to confirm the cut or Esc to cancel. During the cut you can press C to help keep your lines straight.
Some other tools used for modeling are the loop cut and subdivide to split a face into multiple faces. You can also add single vertices and make a face from 3 or more unconnected vertices, it is easy to find beginner tutorials on mesh modeling.
Note that simply cutting a face into another will leave you with an n-gon, which is a face with more than four sides (compared to tri or quad faces). While n-gons can be helpful during the modeling process, as you progress you will find times when they can cause issues. It is always recommended to maintain a quad based mesh as much as you can. In your example that would mean adding at least another two edges.