Pls help me write an algorithm for finding area of 2D and 3D objects

Answer 1

Know the formula, create a casa of shapes, create the different shapes as objects. Then define the formula in each objects of the shape you create, and you're good

Answer 2

One word; "Math".
Just google the different formulars for both 2D and 3D objects. Like the area of a square is length * hight, and a circle is pi * r^2.