There are so many strict and complicated  rules if you want to learn Sado, the tea ceremony, from the ritual point of view. However in order to participate in a traditional tea gathering for your one-day experience in Japan, you only have a few manners to follow.

What to Wear
First of all, you must find out what that particular tea gathering is like ; formal or informal, held in a traditional Japanese-style room or a Western-style room. These important informations will give you a big hint when you decide what to wear.

If the gathering is casual, there are only a few things to remember, that is wearing jewels and perfume is prohibited.

You have to be more careful, however, in case your gathering is rather formal and held in a taritional Japanese tea room. 
As some of the tea bowls and other utensils you handle are fragile and very valuable, you should not wear anything that can possibly do harm to them. For this reason, take off  all the accessories you wear, such  as rings, long chains and even a wristwatch. You must know one can never be too careful enough when handling these items.
Do not bring any smell to the tea room with you because it spoils good fragrance of tea and incense. 
I recommend ladies to wear skirts with the length that covers your knees when sitting on the floor and gentlemen to wear jackets and pants which are not too tight at the knees for your circulation.
You may have already realized that you need to sit directly on the tatami mat for more than twenty minutes before the tea is served.

Even if you are confident of being dressed parfectly according to these recommendations, you had better see once again if your socks or stockings are all right. You must, at least, make sure your tiptoe is not out of your sock before setting off. Participants sitting on the floor in the calm may see much lower objects than you imagine.

What to take
You had better take a pair of white socks and a quire of washi paper called kaishi(懐紙).
We usually wear white socks just before entering the tea room for they are symbolic of purity. 
A quire of washi paper is quite useful. We sometimes use it as a little plate and it sometimes becomes a napkin during the ceremony.