Congrats on starting swimming, for me it's been the one constant for the last 18+ years, and the benefits are amazing. Anyway, I highly recommend doing lots of stroke drills in the beginning. Get the Total Immersion DVD or another swimming DVD and do the drills every workout. Swimming is 70% technique, it's better to learn the right way, then try to correct a bad stroke later. You'll get a great workout just from that. Also, don't try to do heavy yardage in the beginning, if you haven't swam before start off with 800-1000 yards or less. Then steadley add more yardage every few weeks, like 100-200 yds. Anyway, once you get comfortable you can do a combination of overdistance (steady) swimming (e.g. 400 yds) and then add in intervals like 8X50 yds on the minute. There are infinite combinations of intervals, ladders, drills that you can do, just look around and read. The US masters site is good place to start, just cut down on the yardage that they do. A sample workout for a begginer could be:
Warm-up 300 yds.: 4x50 (stoke work), 1x100
2 x 150 w/ 40 sec. rest interval
6 x 50 @ 1:00
1 x 200 timed
2 x 50 stroke
1 x 100 cooldown
1200 total yards
Good luck. Update with your progress.