Start with scooping out as much of the debris as you can as you mentioned. When you are ready to start the pump and chlorinate, you can just use plain unscented household bleach to shock with. Do a lot of reading here on the forum and also on our sister website www.poolsolutions.com. I would also suggest that you go ahead and order a good kit. The one we recommend is the Taylor K-2006 or 2006C which is a better buy. You won't be able to buy it locally but can get it through the link in my signature below. If you go ahead and order it, you'll have it to help you clean up your swamp. After you get most of the debris out, repost and someone here can help you get started.