There are no special requirements for dancing Forro, but relaxed Brazilian style is a good guideline. You should wear cool, comfortable clothing that you can easily move in. Breathable materials are best. Flowing skirts will show off hip movements for women.
The only tricky part is shoes for the forrozeiras (girls who dance forró). Since the followers mostly dance on tiptoes, it is really important to have comfortable shoes that will support the sole of the foot and not slip off the heel. Shoes with heels common in other latin dances are not used, mostly due to the style of dancing but also because it can get dangerous on crowded dance floors. Any flat shoes that will not slip off your feet and allow you to turn easily are good. You can also dance barefoot or in socks.
Forro is a fun party dance so being relaxed and happy are the most important thing!