To get started you will need to download the software. It is free. It is open source which means it is managed by volunteers. Go to www.processing.org and download the software for your computer from there.

When you have downloaded it you will get an icon similar to this on your desktop (or wherever you downloaded it). Next step is to click on it, open it up and start typing your code in. To see the effect you click on the triangle in the circle pointing right (it runs the sketch you have just created). Sketches are the lines of code that make up the whole programme. 

What you get is blank canvas with numbers down the side for each line of code. A sketch will include at least two main elements the void setup() which only happens just once and void draw() which loops continuously.

void setup()
{
  // happens once
}

void draw()
{
  // continuous loop
 }

The curly brackets {} (braces) show where that bit of code starts and finishes. The code goes in between them. They always operate in pairs and if you forget one (and you will) very surprising things may happen. You may have lots of loops with loops and it can be easy to lose track. A way to avoid this is to always put the closing one in as you type in the opening one before you put the code in between. Trust me, it is good practice to do that but only really matters as the code gets more and more complex. The // means a note, a comment, doesn’t do anything code wise but useful to leave notes and explanations for yourself or others