Usage

Starting the editor

There is also the keyboard shortcut [ (open square bracket) to open the editor. 

Choosing a language

Then choose a language from the language menu:

Now you can write your script.  In this tutorial, Jython was chosen as scripting language, but the process is really the same for all scripting languages. 

Running the script

Once you are satisfied with the script, run it.  This does not require saving, but of course you should save your script later when it works. 

Note that while the script is running, the window title shows the tell-tale (Running). 

You can use all of ImageJ's classes right away.  Here is an example that shows a dialog where the user can input a number. 
For details how to write dialogs in the different scripting languages, see Scripting comparisons

Further reading

See the Scripting Help page for an introduction to scripting.  For more information about each specific language, see the Languages section of top right sidebar.