.bas files in VB6 were 'Modules' and you can have modules if you need in .NET as well. A module is basically a Public Class with methods marked as 'Shared'. What this does is allow your code to call the methods in the module without having to make an instantiation of the class.
I would reccomend creating a class as opposed to a module with the following declaration:
Public Class MyClass
Public Shared Function SayHello() As String
Return "Hello!"
End Function
End Class
You can use a module, but to get in the mindset of moving twoards more OOP and .NET methodologies, I would use a class. I believe the module still exists for old VB6 developers as a 'comfort for conversion' and to help feel good in developing in .NET. The code above is equivilent to accessing code in a module and could be accessed like in the sample code below:
Dim MyString As String = String.Empty
MyString = MyClass.SayHello()
As mentioned before, there is no real great conversion path from VB6 to .NET, so some of the code regardless of the container used (class or module) is going to have to be manually updated to reflect the .NET equivilent.
Hope this helps! 