To create a Team account, you'll first need to sign up for a Pro subscription. Then, head to Account Settings > Teams to create your new team.
Once you set up your Team, your personal account will become a Team Owner of the new Team account and will be able to handle billing, add teammates, and post as the Team.
Can I convert my existing profile into a Team account?
Upgrading to Pro will not convert your existing account into a Team profile - all Team profiles must be created following the steps above. We are also unable to transfer shots, followers, or other data from an existing account to a Team profile.
See also:
Adding or Removing Team Members + Owners
Do Team Members get Pro features?