This is probably the most commonly asked question after my clients have booked their session. My advice would to first and foremost wear something that represents you and something that you are comfortable in. You are more then welcome to bring a change of clothes if you want to do something a bit more dressy and then something more casual.
Try and stay away from white shirts or shirts with large logos. I always feel that the simpler the better. Layers look great and make sure you put some thought into cute shoes and accessories too!
For newborns, they always photograph best in their birthday suit! Also, remember to have some and cute hats, shoes, and accessories.
I am always willing to help out too! So just let me know if your struggling with coming up with something for your session! :)