Before you can build your first AppleScript you need to know some basics: what a script-able application is and how to use the Script Editor.
Basically, script-able applications are applications in which the developer has included a set of "instructions" that the Script Editor understands and can use. Some apps are not script-able but most Apple apps are and, of course, Mac OS X itself is script-able.
The Script Editor
The Script Editor is a very simple application, but it allows you to do some very powerful things. The Script Editor is located in your Applications folder (Applications : AppleScript : Script Editor.app) by default. The editor has a simple interface with a prominent text entry area. It also has 5 buttons in the toolbar; for now we will only focus on the "Run" and "Stop" buttons. The run button will run the currently typed script, and the stop button will cancel a running script.