In the pool, start by treading water.
You should be able to tread water for five full minutes without sputtering and gasping, says Marsh. "The phrase 'treading water' implies you're trying to stay up high like a water polo player polo. Focus more on comfortably floating with your head above water without touching the sides or the bottom of the pool." Move arms back and forth under the surface of the water, bringing them in front of you and then slightly out to the side. Your legs should be flutter-kicking beneath you, with your feet relaxed (not flexed). As a rule of thumb, Marsh recommends warming up in a body of water that actually feels warm and comfortable, as a super-cool pool tends to make us tense up. If the water is chilly, get your blood flowing by doing some calisthenics on dry land and vigorously jumping up and down in the shallow end.
Familiarize yourself with the breaststroke.
It's important to master a stroke that can easily get you from one side of the pool to the other, and could conceivably take you from a sinking canoe to the shore. For most of us, this is the breaststroke. It's easy to do, hard to mess up (even when the swimmer is stressed), and takes much less energy and skill than the freestyle or butterfly. Perfect form matters less in this stroke than others, but here's a quickie refresher:
Take it two laps at a time.
Make it a goal to swim from the shallow end to the deep end and back; this will help you develop stamina. Try to wait to fix your goggles or take a breather in the shallow end, where you'll be able to stand to the side of the lane and will be less likely to disrupt the rhythm of faster swimmers. Marsh, who has coached tadpoles, Olympians (32 of them), and recently, a 72-year-old former Navy frogman, says that swimming is experiencing a popularity boom, so it's likely that you won't be the only person in the pool making a long-awaited comeback.