You need a Stellar account to join us. You can create it by adding a wallet (for example, the Stellar Freighter) to your browser. And this will be the thing to do should you decide to join us on Stellar public network.
You don't need a wallet to create and fund your account on Stellar TESTNET. Use Stellar Laboratory to do so. Create and fund your Stellar TESTNET account, then copy your Secret Key and paste it here:
When you press the Store button, we store your Secret Key on this device ONLY. And we update your Stellar account to make it trust our assets.