Go to https://trials.dynamics.com/ Click "Try for free" on the Dynamics 365 Business Central option.
The next page will ask you to sign in with your username and password, then click Next. Once the setup is finished, the tenant will be created and you will see the "Home" page.
Business Central requires that you use a work, or school, email address to sign up. Business Central does not support email addresses provided by consumer email services or telecommunication providers. This includes outlook.com, hotmail.com, gmail.com, and others.