For now you don't have to worry about which imports to use just use these in any applet you write. The next part of the applet is the definition of the applet. The next part of the applet is the first method. A method is a piece of code that does something. It's like a function or an AppleScript handler.
Applets have several standard methods.
The public means that the method can be called by other parts of Java. The void means the method doesn't return any value to whatever calls it. The methods name is init. You'll see an example of a method that has input arguments in a bit. The init method is called when your applet is initialized.