Most women prefer to wear tank tops or a sports bra that's made of a cotton and lycra (spandex) blend when they do yoga. Loose tee shirts and underwire bras are usually a no-no because they have a tendency to get in the way of movement.
For the bottoms, most women prefer to wear tights. These comfortable and stretchable pants will provide you with a full range of movement and allow your instructor to check your alignment. For slower-paced classes, you may be able to pull off wearing long skirts or stretchable harlem pants.