Dress: I recommend good walking shoes, as you want to explore as much of the mountain as possible in comfortable style (most of the walking on Table Mountain is done on rocky paths and the area is large). Bring a light top in summer as winds and altitude means that it can be slightly chillier than at ground level. A BIG must is water and sunscreen. Summer time means that Cape Town is hot and the sun is scotching.
Opening times, ticket prices and weather conditions can be found on (or ask in reception): http://www.tablemountain.net/content/page/rates