For an absolute beginner, my advice is to start on some p5js online editor, such as this 1: 
OpenProcessing.org/sketch/create
Some sketches I’ve got there btW: 
By using such specialized p5js online resources we don’t need to make our own “index.html” file. 
When you decide it’s time to learn how to code your own “index.html” file, you can use more generic JS online editors, such as these 2 below: 
You can also take a look at my sketches I’ve got over there: 
CodePen.io/GoSubRoutine/pens/public