Step 1: Login to windows server using the same windows credential user you used during SQL Server installation.
Step 2: Verify or add the new windows user credential (assuming you are using different user credential to connect to TFS) to be part of sysadmin user group in SQL Server or ask your SQL Server DBA to add the user to sysadmin group. Note: This is required because for every new TFS Team project created, TFS presents 2 option empty database or new database. To create new database the TFS admin user have to be part of SQL server sysadmin group
Login to windows server using the same windows credential user you used during SQL Server installation.
Verify or add the new windows user credential (assuming you are using different user credential to connect to TFS) to be part of sysadmin user group in SQL Server or ask your SQL Server DBA to add the user to sysadmin group.
Note: This is required because for every new TFS Team project created, TFS presents 2 option empty database or new database. To create new database the TFS admin user have to be part of SQL server sysadmin group
If TFS 2012 was installed using the same windows credential, open the TFS Admin console and add or verify the user is listed in "administration Console Users" list.
Step 3: If TFS 2012 was installed using the same windows credential, open the TFS Admin console and add or verify the user is listed in "administration Console Users" list. If TFS Admin console reads "You do not have permission or not administrator" or something along this line, then you need to log out and log back using the correct windows credential used for installation or use one of the TFS admin user and then add the new user credential.
Recommendation: To avoid confusion I would recommend use the same user credential for SQL Server and TFS server. Ex: "domainname/tfs" is local admin to the server, sysadmin in SQL Server DB and also admin user to TFS server.
To make like simple start of by, adding the windows user (intended to use as TFS admin) as windows administration group. Then logout and log back using the TFS admin user to install SQL Server and TFS server.
Hope this helps