This unit is basic geometry. It covers everything you need to know to get started:

Below, it includes Definitions of points, lines, rays, segments, types of angles, types of triangles, angle sum property, quadrilaterals, properties of regular polygons, circles(radius, diameter, circumference,[basic parts; center, chord, arc, tangent]), area and perimeter for triangles, rectangles, circles, volumes and surface areas of simple 3d shapes.