To add a new workspace, navigate to your account settings and choose 'Workspaces' from the drop-down menu in the upper-right corner:

Click on the 'Create new workspace' button:

You’ll see a popup window where you should enter a new workspace name. Once you’re done, click the 'Create and go to billing' button. You’ll be taken to the billing tab of the new workspace.

Keep in mind that workspaces are billed separately so make sure you really need one. Read an overview of workspaces for more information.