A flexible tailor's measuring tape. They come mostly in plastic now, but sometimes you can find a paper or even cloth ones. Definitely do not use a metal carpenter's measuring tape or yard stick or anything of the sort.
Shoes that you will wear with the garment or some that are of the same height. You need to wear these when taking the length measurements.
Undergarments that you plan on wearing with the garment. This may be a corset, bustier, strapless bra or a regular bra. Of course it is also ok if you dont want wearing any undergarments.