I am hardly an expert (and I'm super slow), but I'd just start. Don't over think it. Go to the pool, jump in, and swim for 20 minutes, resting whenever you need to. Once you've gotten back into the groove and are swimming regularly, then I'd look for training plans. I think you'll enjoy it more if you don't start from day 1 with a super structured training plan. Just swim!
For now, I would just say, get in the distance, and do it consistently. If you just do it every now and then (:::hangs head: it's going to be much harder to get your swimming fitness back.
Also, I'm a huge fan of 8 or 10 25's or 50's sprints(wherever your at on distance). These help to get your heart rate up and make the rest of your workout much more effective.