Python program to calculate the area of two-dimensional shapes
Generally looks good so far (I especially like how you set length and width to an inline call to raw_input() -- less code's better).

One suggestion is to only take radius as a parameter to the circle function and instead use the math.pi constant and import math at the top of the program, making the top of your program the following:

import math
def circle(radius):
   area_circle = math.pi * (radius ** 2)
   print " the area of your circle equals %s" % (area_circle)


To add the thing where you keep prompting the user for yes or no until they enter on of them, although I suppose I can't give you code (think that's what StackOverflow is for), think about initializing the response string to something that isn't "yes" or "no" outside a while loop where you keep checking if it's a valid response and asking the user for input...