The first thing you must decide is which thickness of suit is best for you. Generally we would always recommend going thicker rather than thinner if you are unsure. It is unlikely that in the water you will become too hot. In tropical waters consider a full length suit to protect your arms and legs from abrasion and the result stings. The time when you are most likely to overheat is waiting to enter the water, so try to leave suiting up until last and drink plenty of water.
Remember if you plan on deeper diving the water temperature will get colder as you get deeper. There may be several layers of water temperatures, so plan for the deepest part of your dive.
In colder waters you may want to consider layering your thermal protection. Just adding a rash vest under your wet suit as a base layer will keep you much warmer than without. This is also a great tip if you have an old wetsuit that has compressed over the years and is no longer as warm as it once was to boost its warmth once again.
If you recall from your dive training, as you descend deeper the neoprene which the suit is made of will crush and compress, this means that the suit will become less buoyant so you will need to add air to your BCD to remain neutrally buoyant.
After the day or nights diving wash your suit in fresh water and allow it to dry in the shade. Avoid leaving neoprene exposed to direct sunlight for long periods as it degrades the neoprene.