If you are going to practice for the first time we recommend you to wear a sports wear and have an obi belt (optional) with you for the first practice.
Later on you will need:
Iaido gi for everyday practice (or the upper part of the clothes)
hakama for everyday practice (the bottom part)
obi (the belt)
mogito or iaito (the sword replica for training purposes) and a special bag to carry it.
montsuki (ceremonial upper part of the clothes)
striped hakama (ceremonial bottom part)
tabi (socks used by high ranked sensei or for ceremonial purposes)
Iaito maintenance kit (oil and tissue-paper)
Furoshiki (to put the clothes into)
Bokuto (a wooden sword)