This is a basic checklist and if you follow then you can secure your SQL Server instance.

• Disable all guest accounts
• Apply strong passwords to SA account
• Rename builtin administrative account
• Prefer Windows Authentication
• Install Secure Socket Layer or SSL
• Install a Network Firewall
• Isolate SQL Server from your Web Server
• Restrict physical access to SQL Server
• Secure SQL Server data files with NTFS Permissons

Bandagi!