How do I understand the principles of optics and light?

Light behaves as a wave and as a particle. In optics, you treat light as a wave that propagates through different media. First, you need to know how light propagates (Hyguen's principle). The next question is how light behave when propagating from one medium to another, which introduces reflection and refraction (Snell's law). Then, interference and diffraction are characteristic phenomena of light. Lenses and optical systems depend on refraction and maybe dispersion (The spreading of light into the wavelengths that constitute the light because the speed of light in a medium depends on the wavelength). This is almost all that you need to know about light and optics. From where? depends on what level you intend to reach.