Below are some outline notes about some of the topics that you need to know about. These are intended as a supplementary resource and NOT to replace the background reading that you should be doing.
Diffraction: The notes and diagrams should give you a basic introduction to diffraction
Superposition and Interference: Waves can overlap with each other, when they do they join together to form a single wave. It is possible for two waves to, in effect, cancel each other out if they have the same amplitude and wavelength. You need to understand these ideas and the difference between constructive and destructive interference. Some notes and diagrams.
Young's Double Slit experiment: This is a famous experiment that shows how a single light source can show an interference pattern. Notes
Thin Film effects: Young's double slit experiment is one example of interference between light waves. Interference occurs in many more common places. If there is a pool of water with a small amount of oil on the top of it we tend to see a number of colours. This is an example of what is called thin film interference. More here.
The single slit experiment: This shows how light is diffracted. Notes
Other uses for diffraction : X-Ray diffraction is the most common, you are going to have to read up on it but a simplistic approach to the the theory goes a bit like this.