C.B.
My 12 year old started wearing bras last year. We went to Target and got some sports bras. We bought them a little on the large side so they are not uncomfortable. They are basically like a short tanktop with thin bra straps. She also started wearing anti perspirant after they had the health class in school about hormones and periods and body odor (separating boys and girls). They were sent home with samples of products, including some panti liners, pads, small deodorant.
Like others said, if you do not talk to her about why she needs to, some mean girls are going to point it out soon, or some boy is going to tease her.