To become a validator, you need to set up a wallet.
You can get the polkadot.js browser extension from here:
Import your mnemonic as a key into the polkadot.js extension.
First, open the extension. Then, press on the
button at the menu:
Press on "Import accounts from pre-existing seed", then follow the dialog to import the key.
In the end, you should see the address corresponding to your validator key. Use this address when getting the tokens from the faucet.