You need a 20" wheel, tire, and rim strip. You'll probably have some options for what width of tire to get. They start at around 1.95" wide. Most actual BMX frames can take at least a 2.1" in back. There's a degree of preference there. The rim won't care for the most part.
You need to select a rear cog size that both matches the chain length if you're planning on re-using it and/or is a functional combination with your chainwheel size. All the BMX parts sites like Dan's and Albes have extensive guides on this, and there's a question or two about it on here as well.
If you wind up needing a freewheel or cassette cog then you'll likely be buying it separate from the wheel. If you need a one-piece driver (8, 9 or 10 teeth) than typically one selects a wheel that comes with the size you want pre-installed.