Get hold of Michael Kaplan's book Internationalization With Visual Basic (maybe secondhand). It's a goldmine of useful information. I have some peeves with the editing - the index is awful and the chapter order is a bit random - but it's still excellent. There are some free sample chapters on the book's website.
If you are not already familiar with Unimess - the appalling mishmash that is VB6 Unicode support - do read Chapter 6 which is one of the free chapters. Cyberactivex.com also has a good tutorial on the subject.
Finally, do read the International Issues section in the VB6 manual. It's not exhaustive but it's worth reading.
EDIT: see this answer for a programming-language neutral discussion of internationalisation - nearly all relevant to VB6. VB6 Format function is useful for regionally aware display of numbers, currencies, dates and times. CDbl, CDate etc are useful for converting back from strings to the intrinsic types.
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