If you are diving in tropical waters, a lycra suit might do – it will protect you from stingers while not overheating you.
In any cooler water, a neoprene suit will actually help keep you warm as your body loses heat faster in water than in air.
Neoprene is a spongy material made from foamed rubber. It is quite sturdy while maintaining flexibility over a wide temperature range. The thickness of your wetsuit will depend on the climate you want to primarily dive in.
In general, a 1-3mm thick wetsuit works great in 25-29 degrees of water.
For temperatures between 22 and 25 degrees, a 5-7mm wetsuit is most adequate.
If you are diving in 20-22 degree water, a wetsuit of 8-9mm thickness is recommended. In that scenario, you might also want to look at getting a semi-dry scuba suit.