Electric potential and Electric field

Our first experiment in physics 102.1 is mapping equipotential lines. At first I don't understand and don't know what to do in this experiment because we were not yet taught by our professor the concept of electric field and electric potentials. By using the setup shown below, we can determine which parts of the electrolytic tank(dish of water) have equal potentials.

Connecting these points together what is formed is an equipotential line.Getting points with same potential or voltage is not that easy because we get some slight differences and at a certain point the electic potential fluctuates so we have to wait for the voltage to become steady. After having enough equipotential lines, we can get the electric lines of force which is perpendicular to the equipotential lines. And now the problem is how can I plot these lines in the computer.