I would say HTML, Javascript and CSS for sure - that's if you're just designing the front end and not necessarily implementing it or tying it to the back end. For web sites that do more than just display brouchures to the user, then you need to know some back end programming - PHP, Ruby, Java (JSP), ASP (old Visual Basic 6 - sort of obsolete now) or ASP.NET (VB.NET or C#).
That said, there's probably by now all sorts of third party tool kits where you could build a site that accepts credit card orders without knowing anything more than basic HTML - or even maybe not that much i.e. just Dreamweaver etc.