While editing a song, if no text is selected, any changes made in the "Song Formatting" tab (i.e. font, font size, color, weight, alignment, etc.) will apply to the whole song.
Alternatively, when a portion of text from the slide is highlighted, changes to the song formatting will only apply to the selected portion. Note: selection formatting will override the song formatting, and selecting a template will remove all selection formatting.
Clear Selection Formatting
To remove all selection or override formatting, click the eraser button under the "Song Formatting" tab, under the "Formatting" subsection.
To insert a slide break, place your cursor where you would like the break to occur and press Alt + Enter on your keyboard. The text that was in front of your cursor will stay on the current slide, while the text behind your cursor will be moved to its own, new slide.