>What is the definition of a focus of an ellipse

Ellipse is a locus of points the sum of whose distances to two given points is fixed. The two points are called the foci of the ellipse.

>and how do I
>find its location?

First, there are two fo them. Second, depending on the definition of ellipse, you may have to do nothing. For example, if you use the above definition, the foci are given. No reason to find anything.