It's best to wear loose, comfortable clothing. Pads or props needed to enhance or modify poses are usually provided. Once class starts, there's not much talking or socializing because that interferes with concentration. Classes generally start with a series of poses, each one held for a specific period of time. Your teacher will probably move around the room to check each individual's and suggest ways to improve their form.
Classes typically end with a relaxation period to calm the body and clear the mind.
Most offer classes for all levels, from beginner on up. Yoga teachers aren't certified, so make sure you go to a reputable studio. Also, if you have back, knee, or other physical problems, get the go-ahead from your doctor first, since some poses can aggravate injuries. And tell your teacher beforehand about any limitations or concerns you have, so he or she can help you with alternate or modified poses. If a particular pose is causing you pain, your instructor may be able to help you alter it so that you're more comfortable.