How Do I Locate That Earthquake's Epicenter?
To figure out just where that earthquake happened, you need to look at your seismogram and you need to know what at least two other seismographs recorded for the same earthquake. You will also need a map of the world, a ruler, a pencil, and a compass for drawing circles on the map.
Here's an example of a seismogram:
One minute intervals are marked by the small lines printed just above the squiggles made by the seismic waves (the time may be marked differently on some seismographs). The distance between the beginning of the first P wave and the first S wave tells you how many seconds the waves are apart. This number will be used to tell you how far your seismograph is from the epicenter of the earthquake.
Finding the Distance to the Epicenter and the Earthquake's Magnitude
Figure 2  Use the amplitude to derive the magnitude of the earthquake, and the distance from the earthquake to the station. (from Bolt, 1978) 

Finding the Epicenter
You have just figured out how far your seismograph is from the epicenter and how strong the earthquake was, but you still don't know exactly where the earthquake occurred. This is where the compass, the map, and the other seismograph records come in.
Figure 3  The point where the three circles intersect is the epicenter of the earthquake. This technique is called 'triangulation.' 

4. Do the same thing for the distance to the epicenter that the other seismograms recorded (with the location of those seismographs at the center of their circles). All of the circles should overlap. The point where all of the circles overlap is the approximate epicenter of the earthquake.