i love swimming. i used to do just pool swimming, but after doing that for too long, i got bored. lakes and oceans are much nicer, but not so convenient. but swimming in the bay looking at the sf skyline and golden gate bridge... there's nothing like it.
back to your question... i'd say, always warm up with at least 400-500 yards of swimming/kicking going easy at first but slow building up to some fastER swimming to just get loosened up in the water. anything to avoid a shoulder injury. i can't tell you how many of my friends have had shoulder injuries. it makes me feel lucky.
any kind of interval training you can do is good. maybe repeat 50s and 100s on an interval that's maybe 10-15 seconds slower that a pace you can repeat for 10 or so. so, let's say you can reasonably go 1:00 for 50 yards, do something like 10 50s on 1:10. maybe work up to repeating 200s.
pyramid sets on rest intervals tend to work well for me. so maybe something like 100, 200, 300, 400, 500, 400, 300, 200, 100 wit 10-15 seconds rest between each. but if you're just starting swimming, i'd say start with fewer, shorter "steps" and build up as you get stronger in the water, like in anything.
every so often do a longer swim, either by time or by distance. just try to go steady, keping a constant, aerobic pace. not so hard that you feel really out of breath, but slightly uncomfortable.
swimming alone won't get you a "swimmer's body" though. somewhat, but weight training and core workouts out of the pool come in handy too.
if you want more ideas of things you can do in the pool, try checking out these sites.
http://www.beginnertriathlete.com/cms/article-detail.asp?articleid=528
http://www.swimmingworldmagazine.com/swim-cgi/
