The epicenter of an earthquake is located directly above the focus. Since it is right on top the hypocenter, it is usually the area that suffers the most damage.
Seismologists find an earthquakes epicenter by using the seismic waves that the earthquake released. The difference between the times that the P and S waves reach the seismograph will determine the distance between it and the epicenter. The longer it takes for the waves to arrive the grater the distance.