Now that you know where 'Script Editor' is, here is some basic source code, to start with, ...
01. Launch 'Script Editor'.
02. Copy the AppleScript source code below, between the lines of -- Code starts here --' and '-- Code ends here -- '
-- Code starts here --
property tDelay : 180
property tURL : "http://www.apple.com"
on idle
tell application "Firefox" to Get URL tURL
return tDelay
end idle
on run
set tURL to (text returned) of (display dialog "Enter a URL:" default answer tURL default button "OK")
tell application "Firefox" to OpenURL tURL
end run
-- Code ends here --
03. Paste the, above copied, contents - into a new editor window ('Script Editor', 'File, New' menu item, or 'Command n').
04. Save the source code - 'Script Editor', 'File, Save' menu item, or 'Command s'. A drop down sheet will appear.
05. Enter the desired applet name into the 'Save As:' textedit field.
06. Select the applets destination folder.
07. From the 'File Format:' popup menu - select the 'Application' menu item.
08. Click on the 'Options:' 'Stay Open' check box, to add a check mark.
09. Click on the 'Save' button; or, press the <return> or <enter> key.