Go to https://products.office.com/en-us/microsoft-teams and enter the email address you want to use with Microsoft Teams free.
On the next screen, fill in your name and the other requested info, and then click Next.
When prompted, click Let's go to start using Teams free.
Note: It can take up to a minute for the prompt to appear. Be patient, and don't refresh your browser.
Wondering how to get back to Teams free? After you complete sign up, you'll receive an email link that will take you back to Teams whenever you want.