Well, there are multiple ways. First off, if you're using Windows XP or later, you have automatic access to VBScript. VBScript is a toned-down version of VB6, but is still quite useful and is actually a little easier to learn because it doesn't use variable types... just generic variable definitions. If you do plan to learn VBScript, I would suggest getting Notepad++ from Sourceforge (it's free, and a Google search for "Notepad++" will list the site first). Notepad++ highlights your programming syntax so that it will be easier to see if you've made errors.
If you're looking to jump into VB.Net, you're in luck. Microsoft recently made the 2005 version of Visual Studio .Net Express free, and you can obtain it from Microsoft's website... for free.
Personally, I prefer .Net, but I still use VBScript for minor scripting. The only drawback to VB.Net 2005 is that it requires the .Net Framework 2.0 for Visual Studio to run as well as any programs you create using VB.Net (in case you're developing for other computers). This shouldn't be that big of a problem, though, because .Net Framework is stable and a relatively small download (which is also free from Microsoft's site).
Regardless, it's easy to say that you want to learn VB, but you have to be a bit more specific than that. You have to consider WHY you want to learn it... what you want to program with it. Then, read up on VBScript, VB6, and VB.Net and see which ones meet your requirements and what trade-offs you're prepared to make in terms of cost / efficiency / productivity.
From a generic standpoint, I would have to say to just learn VBScript to start with... you can develop it on any Windows XP or Windows 2003 machine, it's a bit simpler than the other options, and it is a good way to get introduced to the language.