Perform the following SQL checks after installing the Server:

Check if Administrators group belongs to sysadmin role

Check if CmdExec role is restricted to sysadmin only

Check if SQL Server is running on a Domain Controller

Check if sa account password is exposed

Check SQL installation folders access permissions

Check if Guest account has database access

Check if the Everyone group has access to SQL registry keys

Check if SQL service accounts are members of the local Administrators group

Check if SQL accounts have blank or simple passwords

Check for missing SQL hotfixes

Check the SQL Server authentication mode type

Check the number of sysadmin role members

Then, you should require Windows Authentication Mode for connections to SQL Server, whenever possible.

You should isolate your server and back it up regularly.

You should assign a strong sa password.

You should limit privilege level of SQL Server Services.

Configure your firewall to filter out packets addressed to TCP port 1433 and UDP port 1434. ports associated with named instances should also be blocked at the firewall.

You should Use a secure file system.

Old setup files should be Deleted or secured.

You should log failed connection attempts to SQL Server and review the log regularly.Enable auditing of failed connections with Enterprise Manager in SQL Server.