RST (PST)

Hi very new to Sage and trying to figure out how to enter my RST- (PST) I sell products and know that I do not have to pay PST(RST)  on products I resell but there are times that I buy items from companies that do charge me RST(PST). I know that I can take the PST(RST) off when I remit my PST(RST) but how do i set this up in Sage? If someone can explain this and maybe send a screen shot of what it would look like I would be forever grateful.