Select View -> Enable Rulers to toggle the ruler display along the top and left sides of your images. It will use your choice of Default Units for displaying measurements.

To change the type of units displayed by the ruler, change the Default Units setting in the Options dialog.

Guides allow you to align content to specific positions on the canvas. You can adjust their position by simply dragging them across the ruler (rulers must be enabled to use guides). If Snapping is enabled in the Options dialog, then content will automatically snap to the guide when you drag it close by.

To create a guide, simply click on the ruler. The guide will appear, and you can adjust its position by dragging.

To remove a guide, simply drag it past the visible edge of the canvas. If you are zoomed in too far to see the canvas edge, simply dragging it past the visible portion of the canvas will remove it.