I'm reading Computer Programming Using GNU Smalltalk, but I want to know with is the best program to write my Smalltalk sources, because I don't know if I use Notepad++, a editor or a IDE, then I want a suggestion for a beginner.
Remember that I'm using Windows XP.
Dolphin provides a community edition of its Windows IDE. Its not too shabby.
Any editor will do: Windows users will use Notepad or Notepad++ or UltraEdit, Unix users will more likely use Emacs or vi.
The latest version of GNU Smalltalk (going to be 3.2) will have a nice IDE based on GTK+, called VisualGST. I find it extremely useful for reading code, but I still use a text editor for writing it.
If you haven't yet compiled GNU Smalltalk, I suggest you use Cygwin. I'll make prebuilt Windows binaries soon, hopefully in 1-2 weeks since it's now at the top of my todo list. It's a bit more difficult under Windows than under Linux to build gst because of the number of external dependencies.
You might want to look at SciTE, which is a free multi-pane code editor. I've used it for Ruby in the past and, while it's not perfect, it's a reasonably decent tool. I think it'll run fine on WinXP.
You'll not want to start with that. You'll want to start with a Squeak or Pharo, possibly the seaside one-click image. That way you'll have an image-based smalltalk with a working IDE. A file-based Smalltalk like GST has its uses, but development should be done in an image where you can browse, inspect and debug.