Every triangle can be circumscribed by a circle, meaning that one circle — and only one — goes through all three vertices (corners) of any triangle. In laymen's terms, any triangle can fit into some circle with all its corners touching the circle.
To circumscribe a triangle, all you need to do is find the circumcenter of the circle (at the intersection of the perpendicular bisectors of the triangle's sides). You can then find the radius of the circle, because the distance from the center of the circle to one of the triangle's vertices is the radius.
This exercise is a nice one to try your hand at with a compass and straightedge or with some geometry software.