Wear comfortable, loose fitting clothing to begin (you are welcome to wear Gi from other martial arts if you already have these to begin with).
Soft kneepads are a must-have.
Beginners train with a bokutō (wooden sword), we can supply on to start or bring your own.
iaidō-gi & hakama (Japanese formal wear) is worn for training (you may also wear Kendō keikogi, gradings however must be in black Iaidō gi & hakama).
After a few months you may move on to a training sword called an iaitō (blunt training blade).
Eventual purchase of your own iaitō can cost $500 upwards (depending on quality), talk to your instructor before purchasing.