Where is the location of most volcanoes?
Most volcanoes occur where subduction is taking place, that is when one plate is subducting under another.
When the subducting plate gets to a certain depth, it loses water content and this helps to melt the mantle rocks around it. This magma can sit in the mantle for sometime as it goes towards the surface.
As the other user suggests, there are lots of volcanoes around the Pacific Ring of Fire. This is true, but the reason for that is there is a lot of subduction going on around the perimeter of the Pacific.
The other place you can get volcanoes is over hot spots. Hot spots are plumes of molten rock that shoot magma into the crust and form volcanoes. Check out How will plate tectonics change the future? for an explanation about hot spots.