I'm starting to learn Smalltalk. What's the best OS (in terms of having more and better tools) and tools for development?
Most Smalltalks are OS agnostic, even more, you can develop on one and deploy on another OS. Such are Squeak and friends, VisualWorks, Smalltalk/X, VA Smalltalk. On the other hand you have a Dolphin Smalltalk, which is Windows only.
About tools, probably the most professional and with best tools is VisualWorks, which is not open source but it has a non-commercial license for free until you start earning money on it. In open source world there is a Squeak fork named Pharo, which is making a good progress exactly on the tools front.
You can use whatever OS you are most comfortable with for the more popular versions of Smalltalk: VisualWorks, Squeak, VA Smalltalk, and Smalltalk/X. There are Windows specific Smalltalks if you really want to tie into that platform: ObjectStudio is Vista Certified, and Dolphin was built for Windows.
As to learning Smalltalk itself - I'm most familiar with the resources available for learning VisualWorks, ObjectStudio, and Squeak.
For VisualWorks and ObjectStudio, visit http://www.cincomsmalltalk.com, and specifically, you might want to check out these pages:
Smalltalk Daily - A daily screencast focusing on an aspect of VW or OS
Industry Misinterpretations - a weekly podcast focused on Smalltalk (the entire spectrum of Smalltalk, not just Cincom's offerings)
Tutorials - There are a number of tutorials on the Cincom Smalltalk site, both in video form and in step by step html page form.
For Squeak, start at http://squeak.org. You'll find links to tutorials, free books, and examples.
Good luck, and feel free to contact me personally.