AppleScript dictionaries can be confusing to people without much programming experience, but I'll attempt to give you an overview on them.
Each scriptable application has a dictionary, which is split into different suites. Each suite is then split into commands and classes. Commands allow you to do actions, while classes represent objects (such as a document) and have properties that you can get and set.
Some commands that you can use with almost any application are found under the "Standard Suite" section of a dictionary, and are close, count, delete, duplicate, exists, get, make, move, open, print, quit, save and set. Some application's dictionaries also have a "Text Suite", which is used for manipulating text.
Beyond that, the dictionaries are application dependent. Often they have a suite containing the application name (eg "iCal suite"), otherwise it can be split into multiple suites (eg Finder, which has "Finder Basics", "Finder items", "Containers and folders" etc). Each of these suites contains commands and classes that are specific to the application, and must be enclosed in a tell block in your script.
As an example, the following script tells iTunes to play the next track of the current playlist. It uses the "play" and "next track" commands from the iTunes Suite in iTunes' dictionary, which must be enclosed in a tell block.
Code:
tell application "iTunes"
play (next track)
end tell
The dictionary for "Standard Additions" lists the standard commands and classes that can be used in a script without a tell block, including display dialog, beep, choose file etc. This is probably what you're looking for.
Here's a simple example using display dialog:
Code:
display dialog "Hello world"
If you're unsure as to how to do something specific, or have any other questions, feel free to ask.