Hi,
the difference from C# is that in F# the compilation does depend on the order of source files and a code can "see" only types/functions/modules declared earlier. This means that if you have "bar.fs" & "foo.fs", the code in "foo.fs" can use declarations from "bar.fs", but not vice versa. (Rreordering files in VS is unfortunately currently a bit painful - you can either edit F# project file by hand or rename the files in solution explorer, which moves the renamed file to the end of the list).

You don't need to create FSI files or mark declarations as public - by default, everything will be public and FSI files and public/private annotations are only useful for hiding implementation (which may be good for a library, but can be done later if needed).

The simples example is following:

 
// 1. file: bar.fs
let someFunc a b = a + b

// 2. file: foo.fs
// open "Bar" module - by default F# uses name of the file with first letter in uppercase
open Bar 
do printf "%d" (someFunc 1 2)

More advanced options are to use "module SomeName" or "namespace SomeName" at the beginning of the source file, which tells the compiler to put everything into a module or namespace with the specified name (the difference is that namespace cannot contain functions and module looks like a class with static members if used from C#).

You can also use nested modules - e.g. define a module "Mod" in namespace "MyStuff", which also contains class "Cls" looks like this:

 
#light
namespace MyStuff

type Cls() =
  // .. some class members

module Mod = 
  // .. some functions in the module

When writing code in F# I usually don't place every class into a single file (managing the order of files would be difficult), but I usually split the project into larger groups of related classes & modules and use a singe file for every group of things. You also need to make sure that classes/functions that recursively depend on each other will be in a same file. This may look a bit annoying, but it can help to make code cleaner (but I'm not saying that C#-like compilation mode wouldn't be nice sometimes :-) ).

Hope this helps!