Triathlon Swim

Swim technique. If you don't have good technique this is the first thing to work on. You can seek out help from a masters swim program. Sometimes even places that teach kids how to swim will also help adults who want to swim competitively. One of the best programs I've seen on triathlon swimming is from the "Tri Swim Coach" Kevin Koskella.
Repetition is the key to becoming a strong swimmer. The only way to improve your swimming is to keep doing it. If you are doing a race with an open water swim you should definitely add some practice open water swims to your training plan - you especially need to practice "sighting" in the water (swimming straight to a location). The good news is that swimming is a low-impact way to build your cardiovascular system.
Swimming with a group can keep you motivated. A group can keep you challenged to improve. A coached group is also a good place to start. A coach can give you feedback about your form or technique.
Upper body strength will help your swimming. As you develop your weight training program, don't forget to work on upper body and core strength. This will definitely help your swimming.
What to wear during the swim. A snug fitting swimsuit - some people wear bike shorts - is best. This makes sure it doesn't catch water during the swim. For lake swims, if the water temperature is 78 degrees or below you can wear a wetsuit. Most people will wear one if they have the option because a wetsuit helps add to your buoyancy in the water and keeps you warm. If you don't have a wetsuit you need to get used to swimming in cold water.
How far do I have to swim to do my first triathlon? This is probably the number one question I get from beginners. It's obviously a good idea to be able to swim at least the race distance (usually 500 meters in a sprint race) before you do your first triathlon. However, if your race has a pool swim don't be worried if you're not sure you'll be able to swim without stopping - most pool swims allow you to hold onto the wall at the end of the pool to rest (just remember to keep your feet off the bottom of the pool - check with the race director before your race).

Here are some of my favorite triathlon swimming tips.