“What shape should teeth be?”

You have to be specific. It's supposed to be a normal shape, if you don't have any decay in your teeth.

The shape a tooth should be is determined by many things, encluding which tooth it is, and how the teeth around it should fit to it. A true cosmetic dentist can create a very natural looking tooth using this and other esthetic information like the proper size to fit the person's face, where it is in reference to your smile, its position to the midline, how it will effect and be effected by your speaking, and what tiny nuances in detail with edge shape as well as layers of color and translucency are needed to get the perfect looking tooth. Choose a cosmetic dentist with experience, and ask to see his/her work in person as well as in pictures (a good dentist will have employees on staff with work that he/she has done).

Normally, the shape coordinates with the shape of your face.