You can't: there are many different shapes with the same area that have different perimeters. For example, if you have an area of 100, the figure could be a 10 by 10 square (with a perimeter of 40), a 20 by 5 rectangle (with a perimeter of 50), or even a circle with a radius of about 5.64, and a perimeter of about 35.44.
You might be able to figure out the perimeter if you know something about the shape. If you know it's a square, for example, then the perimiter is 4 times the square root of the area.
It's also interesting to note that of all shapes with the same area, the one with the smallest perimeter will be a circle. This is why soap bubbles are round: their contents are fixed, but surface tension makes the bubble "try" to minimize the perimeter.