How to write my C++ function so I can call it from C#?

I'll give you an example.

You should declare your C++ functions for export like so (assuming recent MSVC compiler):

extern "C"             //No name mangling
__declspec(dllexport)  //Tells the compiler to export the function
int                    //Function return type     
__cdecl                //Specifies calling convention, cdelc is default, 
                       //so this can be omitted 
test(int number){
    return number + 1;
}

And compile your C++ project as a dll library. Set your project target extension to .dll, and Configuration Type to Dynamic Library (.dll).

Then, in C# declare:

public static class NativeTest
{
    private const string DllFilePath = @"c:\pathto\mydllfile.dll";

    [DllImport(DllFilePath , CallingConvention = CallingConvention.Cdecl)]
    private extern static int test(int number);

    public static int Test(int number)
    {
        return test(number);
    }
}

Then you can call your C++ test function, as you would expect. Note that it may get a little tricky once you want to pass strings, arrays, pointers, etc. See for example this SO question.