It depends on many factors, such as the intent of the ritual, the type of group, the weather, in doors or outdoors, ect.
With most Wiccan groups the answer would be either skyclad [nude] or in a ritual robe and sandles. The robe color can either match the intent of the ritual or could be simple black or white.

My best suggestion

When in doubt, ask. We often have new comers attend “open” Sabbats & Esbats. Before they come we usually open a dialog with them to let them know the basics of “Circle etiquette” What to wear, what we intend at the ritual, things they need to bring, where to stand, how and when to respond and the basic things not to do as well.

If you are attending your first ritual or even just the first ritual with a new group, take the time to ask for a bit of information first. It can be problematic if you do not understand the taboos of a group (some do not allow watches, others don’t like braided hair, others still frown on women in trousers) and it can likewise be a big shock if the group you are visiting work “skyclad” (unclothed) if you are unprepared for it.

A few simple questions can save a lot of confusion. If you are left to a general “nice clothes” then the same sort of clothing you wear to a good restaurant will be fine. The most important thing you can wear is a smile and an open mind.