1st place I'd go is  -> http://processing.org/reference/
Then go to tutorial -> http://processing.org/learning/

It'd be informative to know which is your programming language background.
It's important to compare the "commands" from your previous language w/ Processing.

Know that Processing's main language base is Java, and soon after that, JavaScript!

Found some tutorials on YouTube:
https://www.youtube.com/playlist?list=PLCF0B131E9150F397

Processing is more focused on creating visuals. A "Hello World" sketch is a good idea to make sense of though. It uses println() which will be very useful for debugging any sketch you make in the future when something unexpected happens. Here's a "Hello World":

or this one.....

1. // setup() is the built in function for Processing that gets run first
2. void setup() {
3.   size(200, 200); // Create a window that is 200 x 200 pixels big
4.   println("Hello World"); // Display text
5. }
6. //
7. // draw() is the built in function for Processing that
8. // gets run again and again automatically 
9. void draw() {
10.   background (3, 2, 255);
11.   fill(255, 3, 2);
12.   text("Hello World", 70, 100);
13.   line(110, 10, 10, 110);
14. }
15. //

...or 3D

1. float i;
2. // setup() is the built in function for Processing that gets run first
3. void setup() {
4.   size(200, 200, OPENGL); // Create a window that is 200 x 200 pixels big
5.   println("Hello World"); // Display text
6.   textSize(19);
7.   //frameRate(22);
8. }
9. //
10. // draw() is the built in function for Processing that
11. // gets run again and again automatically 
12. void draw() {
13.   background (3, 2, 255);
14.   fill(255, 3, 2);
15.   line(110, 10, 10, 110);
16.   translate(40, 40);
17.   box(30, 10, 40);
18.   //pushMatrix();
19.   translate(100, 0);
20.   rotateY(i);
21.   text("Hello World", -40, 100, 0);
22.   //popMatrix();
23.   i+=.01;
24. }
25. //

As another geometrist (coming from actionScript), I first played with the 2D primitives: rect(), ellipse(), line() and so on, together with fill() and stroke().