If you don't have a swimming background, then you need to learn to swim in a controlled environment before you move to open water. This may mean lessons, or if you had lessons as a child, then getting into swimming shape. I would recommend swimming in a pool until you can swim (at a minimum) 1.5 - 2x the distance of the triathlon in a single session. For example, if your triathlon swim is 500m, then you should be able to swim 750-1000m.
For the wetsuit, it's important to know if your triathlon has a history of not being wetsuit legal. Over a certain temperate (78F/25.5C in the US), you are not allowed to wear a wetsuit. If it is generally a wetsuit legal swim, a wetsuit is a very good aid, even for experienced swimmers.
For the open water acclimation, get with a buddy either swimming with you or in a support boat of some kind. The water will have poor visibility, taste funny, move independent of you, have waves, etc. It can be intimidating the first couple of times. Go in small increments close to shore before moving further out into the water. Be aware that if you are in a recreational area, there may be restricted swimming areas and/or motorized watercraft zipping around that may not see you.
Practice, practice, practice. The biggest thing I see in triathletes that are not good swimmers is that they tend to wander side to side and get way off course. A way to test this is to swim in a lane with ropes, and close your eyes. If you keep hitting one of the ropes, you have a stroke imbalance somewhere. Also learn/practice sighting, or looking ahead on the course to pick out your target. You should be able to do this while still swimming, you shouldn't have to stop. If you do tend to wander, you may have to sight more often.
A proper fit on the wetsuit is critical, as if it doesn't fit then it can hamper your swim, chafe, bind, etc. Also, some people tend to feel claustrophobic the first couple times in a suit as they are very restrictive. If this is the case, you can look at overall (bib john) type suits, or leg only.