How can I add a new top-level project to TFS?

Follow the instructions provided in the documentation at https://www.visualstudio.com/en-us/docs/setup-admin/create-team-project

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You must connect from a Visual Studio client that is at the same version level as TFS.

That is, you must connect to TFS 2015 from a version of Visual Studio 2015, for example.

You can create a team project from Team Explorer after you have connected to an on-premises server. If you don't see the Team Explorer pane, open the View menu and select Team Explorer.

1. Connect to the server and team project collection where you want to create your team project.

   Tip: If you are running Team Explorer from a server that hosts SharePoint Products and SQL Server Reporting Services, you might need to run Visual Studio as an administrator. If it's your first time connecting to TFS, you’ll need to add TFS to the list of recognized servers.

2. After you've created your team project, you can add repositories. From the Team Explorer window, click the drop-down arrow at the top right, below the search field.

3. Select "Projects and My Teams", and then click "New Team Project...".

   Note: If you are on a different version level, the "New Team Project" option will not exist!

4. Enter a name and a description for the project in the wizard that appears, and then click Finish.

The top-level project now exists with the name you provided.