To construct a circle that circumscribes to a triangle, you would have to construct a circle that where all vertices of the triangle are on the circle. To do this you would have to construct the perpendicular bisectors of each side with your compass and straight edge. Comment on this answer if you are unsure of how to construct a perpendicular bisect (it's a long fundamental process to describe, and I wouldn't want to lecture you one something you already know). Once you have done so, set your compass point on the point where all perpendicular bisectors intersect (they should intersect in ONE point, if not you will have to redo it). Set your other compass lead on one of the vertices and spin away! If you have done this correctly, you should hit all three vertices when spinning your compass. Hope this helps!
Fun fact: the point where all perpendicular bisectors intersect is called the circumcenter
