General Tcl/Tk programming and introduction: See Brent Welch's unbelievable book Practical Programming in Tcl and Tk. Due to Brent's generosity, you can even read and print the older editions and selected chapters from the current editions at http://www.beedub.com/book .
Downloads needed to develop in Tcl: See http://www.tcl.tk for TclPro 1.4.1 for all operating systems, plus almost any add-on package you could ever want. TclPro contains the 2 interpreters (Tclsh and Wish) version 8.3, plus an excellent interactive debugger and a suite of helpful tools and libraries. Version 1.4.1 was released to the public. However, as of mid-2002, it looks like ActiveState is taking over the TclPro product as a commercial product. Remember you can always get the 'standard' interpreters for all operating systems from http://tcl.sourceforge.net because Tcl is open source software.
Editors with syntax highlighting, etc: For MS Windows, I like the inexpensive commercial product TextPad at http://www.textpad.com. Currently the cost is $27 US per license, and you can try before you buy. Be sure to get the Tcl syntax definition file from their web site. TextPad is the most feature-rich editor for MS Windows I've ever seen, and has the ability to emulate Microsoft editors' behavior. You can use it as an IDE for Tcl/Tk development by interfacing it with the interpreters and your other tools. For Unix/Linux, and maybe even for MS Windows, try Nedit at http://www.nedit.org. It's free under the GNU General Public License. It also does a good job of making MS Windows users productive right away.
Tools you'll probably want: The first thing most VB programmers want is to hit an ODBC database. Go get the TclODBC 2.2 package from http://www.tcl.tk . It's a DLL for Win32 that hooks you into all ODBC data sources and drivers. It comes with documentation, and there's a minimal example above. Note that it may or may not be portable to other operating systems, so you might want to wrap all your calls to it into procedures. That way you can port your code to use other libraries later. Regular expressions are almost a powerful programming language of their own. Accordingly, they take some time to master. The simple Tcl program 'Visual RegExp' has helped me tremendously with that. Get it at http://laurent.riesterer.free.fr/regexp . There are also several packages available for hooking Tcl to the world of ActiveX, so you can automate MS Office applications, etc..
Essential help topics: Once you have TclPro and its help file, go to its index and visit the 'Tcl' topic. There's a concise summary of the language's syntax rules, and the substitutions that drive it. Also be sure to hit the 're_syntax', 'tclvars', 'tclsh', and 'wish' topics. These are apparently translated from the Tcl man pages on Unix/Linux, and are some of the best texts I've ever seen for WinHelp, if you need reference material. I don't recommend reading this help file as your first introduction, but it is an excellent reference while programming.
'Start' menu items: Once you have TclPro installed, you should look at the 'Start' menu for TclPro, and check out the 'Incr Widgets Reference' and 'Widget Tour'. These show the built-in GUI capabilities of Tk with the actual Tcl code required to use them.
Advocacy (how to convince your management to use Tcl/Tk): A wealth of advocacy information is available at http://www.tcl.tk .