The standard Script Elements (Scene Heading (slugline), Action, Character, Dialog, Parenthetical (wryly), Transition and Shot) are found in the Script Element dropdown in the Script Editor Toolbar. Selecting one of these script elements from the drop down will apply the appropriate formatting to your text, conforming to the industry script standard. This drop down menu is best used for formatting large portions of text. There are also numeric keyboard shortcuts that have the same effect.
If you want to reformat some text, place your cursor in the block of text you wish to change and select the appropriate format element.
The Tab and Enter keys provide context-sensititve formatting your script elements - for example, pressing Enter after typing Dialog adds a new line and changes the element to Character. Refer to the keyboard shortcut article for details. Note that the uncommon elements (i.e. Transition and Shot) are not in the element sequence, but can be applied from the element dropdown menu or by the numeric keyboard shortcuts.
The status bar at the bottom of the application tells you what will happen if you press Enter or Tab.
If you made a mistake and want to cycle backwards through element selections made previously, select SHIFT + TAB.