You don't necessarily need a special VBScript editor in order to code in VBScript - you could just use a normal text editor. However, a special VBScript editor will probably make your life easier, and enable you to code your scripts much faster.
If you'd prefer not to use a text editor, you can check out these HTML / VBScript editors:
OK, to be precise, most VBScript editors aren't solely VBScript editors. Often, they will be HTML editors with VBScript support. This generally means that they will include syntax highlighting for VBScript (ability to recognise VBScript and color code/format it for you). It may also have a toolbar with options that are specific to VBScript. Given you will probably be coding HTML at the same time, it makes sense to have an editor with HTML support anyway.
Regardless of whether you have a HTML / VBScript editor or not, you still need to test your pages in a web browser. As mentioned previously, Internet Explorer is your best bet for this when running VBScript on the client side.
OK, so you've got yourself an editor? Let's move on and learn some VBScript!