You want to create a quad. Create a surface with 4 vertices, at (-1,-1), (-1,1), (1,-1) and (1,1). Make two triangles that divide the quad in half. You can then easily make irregular quads by moving the vertices using VertexCoords().
Look up the manual's sections on surfaces, particularly AddVertex() and AddTriangle().
Look up the manual's sections on surfaces, particularly AddVertex() and AddTriangle().