There are a lot of things to consider when sizing your wetsuit. Often you can get combo suits that having varying thicknesses for different parts of your body. This may be a 5mm torso, 3mm arms and 4mm leggings.
People find that different places need varying protection, so you can mix and match thicknesses for the most comfort.
Wetsuits also have different styles to choose from. As an example, "shorties" are just a torso cover that has very short sleeves and legs. A "steamer" covers everything except your head, hands and feet.
You can also buy numerous accessories for wetsuits, such as gloves, hoods, and shoes. When you head in to buy your wetsuit, talking to a knowledgeable staff member at the store will really help you find everything you need.