How can I set up the distance between the curve line (pixel) ?

I have used the function “GL_QUADS” within the “openGL” on the VC++ to draw a curved surface.
And finished the curved surface, but had a problem:
The distance of curve line to line is too large.
How can I set up the distance (pixel) between the curve line ?
Is there any problem when I use the openGL or VC++ ?
Or is there any function about how to solve this problem ?