You can access the Ruler and Stencils by clicking on the 'Stencils' icon in the lower central tool panel. From here, you can access the ruler ( click on 'Rulers' then click on the ruler to drop one on your canvas ) or stencils from the other categories. If you're using the ArtRage Starter Edition, the ruler will still be available but the stencils will not, as these are only available in the Full Edition.
If you're new to ArtRage, I'd recommend taking a look at our tutorials page at www.artrage.com/artragetutorials.html and downloading our quickstart guide from that page. The manual, available in the ArtRage help menu, goes through each of the tools and functions and explains how they work. You can also take a look at our Tips and Tricks section in the forums here.
Also though, you can jump right in and play with it. Choose a tool from the left hand side. Pick a colour and lightness on the right hand side, then splash some paint ( or glitter, chalk etc! ) around and get a feel for how things work. I hope that helps!