First of all: what is a covalent bond? It is when electrons are shared between atoms. However, when the shared electrons are not equally shared, it is called a POLAR Covalent bond. e.g. Fluorine binding to Carbon, the shared electrons are not exactly in the middle of the two atoms. Because Fluorine is a smaller atom and has more protons and it brings electrons closer to it. So, this is a polar covalent bond. For NH3:

1. Draw the Lewis structure. N is bonded to 3 hydrogen atoms (H), and has a pair of unshared electrons.
2. Use theory to determine the shape of NH3. The electron pair geometry is tetrahedral but the molecular shape is TRIGONAL PYRAMIDAL.
3. According to three dimensional structure, the molecule has a pyramid shape with hydrogen atoms ‘below’ the nitrogen atom. The nitrogen atoms draw the shared electrons in the bonds close to it, so the nitrogen has a PARTIALLY NEGATIVE CHARGE. This makes a NEGATIVE DIPOLE, because it has electrons near it.
4. Hydrogen has a partially positive pole, because the electrons are farther away from H.
5. Therefore, for NH3, the bottom (that has H atoms) is partially positive, and the top part (the N atom) is partially negative.
6. THIS IS WHY AMMONIA IS POLAR. OR THE BOND POLARITY OF NH3 IS POLAR.