Go to https://www.braintreepayments.com/ and click the create account button.
Add the desired login details you want to use for Braintree.
Creating the account will provide you access to your control panel, you need to make sure you have verified your email address with braintree through email.
Now you can go ahead and complete your account information.
Now that your account is set up you need to connect your app with Braintree in the payments section. Sign in using Braintree and authorize access. Your merchant ID will be filled out if you have connected your account correctly.