By default, my Visual Studio installation already sorts System usings first.
But you can change it. See here (check the note).
Go to Tools -> Options -> C# (I assume you are working on a C# project) -> Advanced. Then check the box saying "Place 'System' directives first when sorting usings"
EDIT: For recent versions, MS moved the documentation to a different page: see here (check the Organize usings section).