writing the equation of my own ellipse

I have drawn an ellipse with the origin as the center and the major axis is horozontal.

the major axis is 16 units long and teh minor axis is 4 units. I would like to know that the foci are, how to write the ellipses equation and the equation i can use to put it into my graphing calculator. Please help! i was not taught this in class and I am having trouble teaching myself.