For what it's worth. I started with a similar situation 2 years ago. Following the advice here my pool is now clean, stable, and a pleasure to use.
Get as much of the muck out with a net. You will be working blind but anything you get out will help. Once you have the big stuff out if you can vacuum to waste it will help a lot to. You will have to replace some water if you do that and local water restrictions may make it impractical. If you can't it doesnt hurt, it just will take longer to get it cleared up.
Have patience with the process. Nothing the pool store can sell you is actually more effective than the methods recommended here. In the first week you will see a huge improvement. In the next 3-4 weeks your water will truly sparkle. The transformation is hard to believe.