For riding in hot weather, you need a good quality jersey and shorts designed to wick perspiration away which helps to keep you cool.(they don't need to be the most expensive)
In contrast, clothing made from cotton will become saturated and will no longer remove perspiration. Even socks can make a difference in your comfort. Gloves are always a good idea, but in summer, (fingerless) gloves can help you keep a grip on the bars.
Water, drink a lot of water. As you dehydrate, your body will not be able to cool properly. This can lead to a dangerous situation if you continue to push yourself in the heat with out proper hydration.