How do I download Visual Basic 6.0?

VB6 is no longer available. You don't need VB6 for this.

It is unclear to me what you're supposed to do your VBA coding in but it sounds like you're trying to use Excel or Access. In that case VB6 won't help you anyway as you cannot run VB6 code in VBA. VBA is a subset of what is available in the full VB.

Providing a UI for getting a date is built into the OS. VB6 and other languages normally just expose that functionality. Without a better understanding of what application you're using and how you need to collect this it is difficult to make a recommendation.

As an example, if you were to be creating a form in Access you could simply drop the standard Date Picker control onto the form and the user can either type in the value manually or click the control to use a calendar. If you create the form using the Form Wizard and point it to a table that has a date column then it can auto-generate this for you. No custom coding is required.

If you wanted to do this by hand then open the form in the designer. Right click the textbox field that contains the date and view its properties. Under properties is Show Date Picker. Set that and it'll auto-generate the control for you.

If you want to trigger the date picker on clicking a button or similar, non-standard, approach then you'll need to write a macro to do it. Note that macros are generally disabled in newer versions of Office for security reasons.