I do 3 summer time sprint triathlons per year. I run 30 to 50 miles, cycle 60 miles and do 4 swim workouts (20 minutes each) per week.
The swim is the critical part of the triathlon for most new triathletes. Make sure that you are capable of a half mile swim in open water. If you can do that then you are in good shape.
Once you have signed up for a sprint triathlon, here are some tips that helped me. I learned them the hard way:
Start the swim off to the side of the field of swimmers to avoid congestion. I’ve noticed that a lot of swimmers start too fast and then seem to stall out. If you are directly behind them, then you have to work harder to just get around them. A start way to the left or right of the field of swimmers will avoid the congestion. You will swim a slightly longer distance but it is well worth it. Also very important is to start your swim slowly so that you don’t get out of breath. It is much harder to get your breath back in the water than on land.
After the swim concentrate on making a smooth and quick transition to the bike. Once on the bike put everything that you have into it (all of your concentration and all of your energy). Then survive the run as best as you can.
If you can afford a full length triathlon wetsuit consider getting one. I swim faster in one because it helps my body to ride high in the water kind of like a surfboard.
Good training!