Describe the shape of earths orbit and explain how earths position relative to the sun changes...

Earth has a nearly circular orbit around the Sun, with an eccentricity of only 0.0167.

Because of this, our actual aphelion (farthest from the sun in our orbit) and perihelion (closes to the sun in our orbit) are negligible.  It's our inclination of axis (about 23.5°) that causes our seasons.

However, there is a system of regular orbital oddities that fall into what are called the Milankovitch cycles.  This does involve our very slightly elliptical orbit, the obliquity of our ecliptic (plane of orbit around the Sun), axial tilt and precession ("wobble") that bring us abnormally close to, or abnormally far from the sun.  This brings in very warm ages, and very cold ages.  They are not just calculated, but observed, so we know that the cycles actually happen this way.

However, in your lifetime and mine... none of that matters.  Just the axial tilt and the seasons.