IMHO, this is the best approach to swimming instruction and technique.
http://www.totalimmersion.net/
The advice you are getting on this thread is the typical old school approach -- go swim 10, 000 laps and eventually you'll 'get it.'
A swim stroke is a complex blend of balance, leg, arm, body rotation and breathing.
Terry Laughlin, recognized that energy efficiency and hydrodynamics were the key. He developed a series of drills to allow a beginner to isolate the components of the crawl stroke and build the swimiming motion one component at a time.
It starts with balance. The human body is not designed for the water.
The top half of you body is buoyant -- because of your lungs. The bottom half of your body (legs) is dense and tends to sink. This is why you are so exhausted after one lap. Your legs tend to sink, dragging your whole body under. Instinctively, you react by frenzied, rapid kicking. The kicking motion elevates the legs back up to the surface. But, of course, this is anaerobic and can only be sustained for a brief period.
What this approach does is teach a method that develops a subtle balance around the hips, so that legs will remain horizontal and not sink.
Once that is mastered, the second step is a rotation of the body. That rotation enlists the major muscles of the body into the swimming motion and provides a rhythm that your body will instinctively embrace giving you ample opportunities for breathing, long efficient arm stroke and slow, minimal kick.
This method is favored by triathletes (that's how I got into it -- it conserves leg muscles for the cycling and running) and is taught to all applicants to the Navy SEAL program.
Regards
Coburn