To show or hide rulers, choose View - Ruler. To show the vertical ruler, choose LibreOffice - PreferencesTools - Options - LibreOffice Writer - View, and then select Vertical ruler in the Ruler area.
The margins of a page are indicated by the filled areas at the ends of the rulers.
Indents are adjusted with the three small triangles on the horizontal ruler.
To change the left or the right paragraph indent, select the paragraph(s) that you want change the indent for, drag the bottom left or the bottom right triangle on the horizontal ruler to a new location.
To change the first line indent of a selected paragraph, drag the top left triangle on the horizontal ruler to a new location.
You can also double-click anywhere on the horizontal ruler, and adjust the indents in the Paragraph dialogue.