The beauty of running is that you can get started with very little investment. A comfortable pair of running shoes, suitable running attire and appropriate training is all you'll need to get started in the Great South Run.
Footwear - It is not recommended to wear a brand new pair of trainers before running the Great South Run. Wear shoes that you are familiar with, along with socks that you've worn before. Many sports manufacturers offer a large variety of running trainers suited to different tastes and needs. If you can visit a local running specialist, they may be able to help you choose the best pair of trainers suited to your running style and build. They may even be able to offer good local running routes, and help you along your training.
Clothing - If you don't want to spend a lot of money then a basic pair of shorts and a t-shirt will be suitable, providing the weather is dry. However, for those looking to invest there are many sports manufacturers offering specialist materials designed to keep you dry. During the Great South Run, you're almost guaranteed to sweat. Wearing specialist materials will help your body keep cool whilst offering good "sweat management."
Additional Equipment - Many individuals like to listen to music whilst running, with a large variety of small lightweight music players suited for running now available on the market. Be sure to remember that you will be less aware of your surroundings when listening to music, and this may hinder your experience of the Great South Run if you can't hear anything!
Be sure to have sun screen or adequate sun protection available if the skies decide to clear on the big day. Regardless if it feels cold, the sun can still damage sensitive skin when exposed for a long time.