Display normals of a geometry

I have a rectangle which is created by 2 triangles and it is in x-z plane, and i have object on it. Now, the normals of two triangles (face normals) are y axis i.e. (0,1,0). I want to display normals of two triangles for which i need two points to draw a line to show normal. Intuitively if i take center of a triangle as one and (0,1,0) as second point of a normal then both of the normals will converge to single point. I dont think it is correct as the normal should be exactly perpendicular to the surface. We can do a trick and take one point as a center and other by increasing y coordinate keeping x and z same, but what if we have curved surface and not a plane board? What about the cylinder made of triangles? How could we display normals as a lines of cylinder? I guess i am missing very basic concept here, please help me to understand it and tell me how to calculate the two points of normal.

Take the center of the triangle as one point, and center of the triangle plus the normal as the other.