For any two dimensional shape with straight sides, it's the sum of the length of its sides. For example, a rectangle has four sides. There are two sides for its length and two for its width, so the perimeter is twice the sum of ith length and width.
For a circle (which doesn't really have any sides), the perimeter is just its circumference (which is about 6 times its radius). For a shape with mixed straight and curved sides the perimeter is more difficult to find because, for each curved part, you need more information.