As has been previously noted, you could place the isosceles triangle anywhere on the coordinate plane. But, you might want to consider placing it in a position where it would be easy to determine the coordinates, and where the coordinates of the vertices involve the smallest possible number of different variables.
Here's one possibility. Put the vertex of the isosceles triangle on the y-axis, and the base of the isosceles triangle along the x-axis. Then, possible coordinates might be (0, a) for the vertex, and (b, 0) and (-b, 0) for the endpoints of the base. You can easily verify that the distances from (0, a) to (b, 0) and from (0, a) to (-b, 0) are equal.