I'm sure the attorney will tell you only to answer exactly what is asked. Don't elaborate. I'm a talker, so that was the hardest part for me. A "yes" or "no" question requires a simple yes or no. And answer anything else that is asked as simply as possible. "Did you see the car?" Requires a yes or no, that's all. "Where was it?" requires "in the parking lot", not "in the parking lot, but it looked like he was leaving." Don't give them any more than they ask. That's pretty basic stuff, and I think all attorneys advise people in that way.
And you can say that you don't remember. Or that you don't know. But again, just say those words, nothing more.
I've testified and I wore black. No one told me anything about what to wear or not wear, but I think you should follow the attorney's advice.
The other thing to remember is not to get emotional. I don't know how emotionally invested you are in the case, but no matter what happens or what anyone else says, stay in control. (No screaming, no yelling, no gasping - In other words, don't be like some QVC hosts!) :-)
eta: I just wanted to add that I understand why you're nervous. But just remember that your friend's attorney will be right there and will come to your aid or object if need be. All you have to do is answer questions to the best of your knowledge. This is just another day at work for your friend's attorney, and he won't leave you hanging or let anything go wrong.