Currently you need to wait until a testnet is started - and announced as a topic here on the forum. When that happens there are instructions in the announcement. To see what is involved, take a look at the last testnet announcement: [ValentinesNet] [14/2/24 Testnet] [Offline]
You can also run your own local testnet on your own machine, but that is intended for developers. You don’t need to do this, but details are here.