Bike degreaser + brush + possibly a flathead screwdriver to scrape off solid muck.

Expect to run it several times if the chain is in a bad state. It may be more practical to just get a new one, because cleaning those things can be quite a challenge, and being very dirty, they may just be worn to the point of needing a replacement anyway.

Should you choose to get a new chain, make sure to clean your other components before putting on the new one (chainrings/cassette/jockey wheels)