To show/ hide the rulers choose View → Show Rulers or use the use the shortcut
alt/option + r.
Guides are horizontal and vertical lines that you can place in the Viewport and use to position Shapes.
To create a Guide simply hover your cursor over a Ruler and
click + drag into the Viewport. As you drag, a thin red line will appear along with a number indicating its X or Y position. Release the mouse to place the Guide.
To remove Guides just
click + drag them back over a Ruler.
Use the View → Show Guides menu option or
cmd/ctrl + ; to show/ hide Guides.
You can snap to Guides by enabling Ruler Guide Snapping in the Viewport Snapping options.