If you're new to Processing, start with an example:

Mouse interactions in 2D |  | Mouse Arc Tangent
Flocking |  | Rotating Arcs in 3D

Run the example; watch what happens. Modify the code. Select a keyword and navigate to Help → Find in Reference to learn about it. Open File → Examples for hundreds more.

If this is your first time using a 4.0 release, you can read about what has changed since 3.0 and earlier releases.
Or if you're familiar with 4.0, you can read about revisions and updates in recent 4.0 releases.

Be sure to visit Tools → Theme Selector to choose your own color scheme for the environment. Not fond of sketch_220809a? You can also select an alternate sketch naming method in the Preferences window.