The following are some some links to sites that can help you better your graphics programming skills.

Educational Sites

BetterExplained
Don’t just memorize math, understand it!
betterexplained.com/

glslsandbox.com
Great place to practice writing pixel shaders.
www.glslsandbox.com

shadertoy.com
Another great place to practice writing pixel shaders. This site also makes it easy to use textures, video, and sound in your shaders.
www.shadertoy.com

iñigo quilez’s Homepage
The creator or shadertoy has some really great
articles that will get you up to speed!
www.iquilezles.org/www/index.htm

Marmoset Toolbag 2 – PBR Theory
Some great info regarding the theory behind Physically Based Rendering.
www.marmoset.co/toolbag/learn/pbr-theory

The Book Of Shaders
Very nice tutorial on how to write fragment shaders.
patriciogonzalezvivo.com/2015/thebookofshaders/

open.gl
Great place to learn OpenGL
https://open.gl/introduction

Useful Web Apps

desmos
A no-nonsense very easy to se online graphing calculator.
https://www.desmos.com/calculator