import turtle

for i in range(4):
    turtle.forward(100)
    turtle.left(90)

Then build some simple functions, like 'square':

def square():
    for i in range(4):
        turtle.forward(100)
        turtle.left(90)

square()

Then add arguments to your functions:

def square(size):
    for i in range(4):
        turtle.forward(size)
        turtle.left(90)

square(100)
square(50)

And so on. At each stage, you can see what's happening.