Your 18-55 is certainly sufficient with the proper lighting technique, but if you want to include detail shots from closer up than the 18-55 can shoot, then you may want to consider a macro lens, such as the EF-S 60mm f/2.8 USM Macro or EF 100mm f/2.8 USM Macro (there's also an L version, but it's much more expensive). Macro lenses tend to be the sharpest lenses in any lineup.
However, a new midrange macro lens is typically in the US$400-$600 price range, and lighting gear can cost considerably less and have a much larger impact on your photos. And given that you're probably going to be using most of your images for web delivery, which are usually small sizes, the sharpness edge of a macro lens might make no difference at all. And flash will let you shoot around f/8, which can typically make a kit lens look a lot more expensive than it is.
Learning lighting, Strobist-style, with off-camera hotshoe flashes, in particular, can be done for very low cost, even with multiple lights. And unlike a simple light tent with continuous lights, working with lights on stands with modifiers and radio triggers gives you much more working room and control over the direction, amount, and quality of light.