If you're in the U.S., you go to your Wallet settings, turn it on, and follow the instructions.How do you get an Apple Cash account?
If you're not in the U.S., you basically can't, as it's not available in other countries. There are workarounds if you have a sufficient U.S. "financial presence" but that includes not only a U.S.-based bank account, but also U.S. identification like a driver's license or state ID, which most folks aren't going to have unless they're actually U.S. residents (although Apple did accept my NEXUS card).