Question

Draw an orbital overlap diagram to represent the bonding in ammonia, NH₃∙

Answer 1

Are you also required to draw the M.O diagram because that will help a lot more.
But anywho, you need to just recognize that N has p-orbitals and H has s-orbital.
Use an x,y,z-coordinate plane to help you draw this.

Answer 2

Don't forget about the lone pair electrons on the nitrogen.