Take it step by step.
1, put points on paper.
2, work out how to step from the point with the smallest y-value to the next point along the perimeter in the counter-clock direction, what comparisons and conditions you need to make when selecting the next point, think this out graphically and write down the steps in pseudo-code on another paper.
3, progress to that next point and do the same for selecting the third point.
4, from this you should be able to work out a loop for this.