I am a beginner and have been given a Sigma zoom 28-80mm f/3.5-5.0 macro lens. I would like to try this before paying out for a more expensive macro lens but it will not fit my Nikon D5000. Could any one tell me which lens-mount adapter do I need to buy?
Sigma makes their lenses in multiple mounts. You need to find out which mount it.
If it had a Nikon F-mount, then you would not need any adapter and you would be able to use the lens in manual focus mode as your D5000 does not have an AF-coupling at the mount.
As it does not fit, it must be another mount. In this case you need the whatever-mount to Nikon F-mount adapter. The big catch is that not all such adapters exist because many are not even physically possible to be built. For using some incompatible lenses, some people have gone to the extreme of cutting out part of the camera body to make it possible!
Tangential to your question is that Sigma does not use the term macro as other manufacturers do. The use it more loosely and, in the case of their zoom lenses, none of them is actually capable of macro photography. With a Nikkor Micro lens, you would be able to take true macro photographs. Since you say that you would like to try before buying I suggest renting a macro lens. These cost around $50 per weekend for Nikon mount.