Circumcenter  Centriod  Incenter
 all four points together
Angle bisector instructions     Perpendicular bisector instructions
Median instructions    Altitude instructions
Period 3     Period 4     Period 5

The orthocenter is the intersection of the altitudes of a triangle.
To construct an altitude refer to your textbook, bottom of  page 56.
Remember, you may have to lengthen the segment in order to construct the first arc.