You need a Braintree account to integrate the payment gateway into your site. Let's create a sandbox account to test the Braintree API. Open https://www.braintreepayments.com/sandbox in your browser. You will see a form like the following one:

Fill in the details to create a new sandbox account. You will receive an email from Braintree with a link to complete your account setup. Follow the link and complete your account setup. Once you are done, login at https://sandbox.braintreegateway.com/login. Your merchant ID and private/public keys will be displayed like this:

You will need this information to authenticate requests to the Braintree API. Always keep the private key secret.