Hello World is a computer program that prints the line "Hello World" somewhere on a display device. It is a tradition in teaching computer programming to start with a Hello World program. First, click on the Control button in the top left panel.
The program blocks under Control are used to control the flow of the program. Control flow in a program can consist of events, loops, and conditional statements which determine when commands are executed, how many times they are executed, and which ones are executed. We will deal with loops and conditional statements later. We start with events because events are triggers that start things happening. All event blocks in Scratch look like this:
Another type of block is the statement block which we already have experience with. All commands are statement blocks. All statement blocks in Scratch are shaped like the following (double-notched) and can be stacked into a series of statements.
Blocks can be different colors depending on the category of statement to which they belong. The possible colors/categories are shown in the top left panel. You will notice that the notches on the bottom of event blocks match the notches on statement blocks. When a series of statements is attached to an event, triggering that event causes the series to be executed one-by-one in order.

The most fundamental event in Scratch is the Green Flag. Clicking the Green Flag in the top right corner is equivalent to starting a software program. Using a single-click, drag and drop the following onto the Scripts panel:
Now select the Looks category. Drag and drop the "say Hello!" block onto the Green Flag event that is current in the Scripts panel in order to attach it.
Now change the text from "Hello!" to "Hello World!" A single click of the green flag in the top right corner of the stage will run the program:
If everything was done correctly, you should now see the following on the Stage: