For Sound Bath Meditations it is best to come in loose fitting clothes. What you might wear to the gym or a yoga class would be perfect. You will be laying down for about 50 mins so you'll want to feel comfortable. You will want water to drink - before and after. You will want to bring a mat. Since you will laying down, you will want a thicker style mat than your usual yoga mat. I recommend purchasing, at least, a 1" thick pilates/exercise mat. An eye pillow, is also highly recommended to block out the light and help keep an inward focus.
Some people bring pillows, blankets, sleeping bags.
I will give you our recommendations for what to purchase below