os: windows 7 selenium versión 3.0.1 mozilla firefox: 48.0.2

Traceback (most recent call last):
  File "C:\Users\LENOVO\Desktop\kk2.py", line 4, in <module>
  driver = webdriver.Firefox()
  File "C:\Python27\lib\site-packages\selenium\webdriver\firefox\webdriver.py", line 135, in __init__
  self.service.start()
  File "C:\Python27\lib\site-packages\selenium\webdriver\common\service.py", line 71, in start
  os.path.basename(self.path), self.start_error_message)
  WebDriverException: Message: 'geckodriver' executable needs to be in PATH. 

por favor brinde información paso a paso a fondo por favor, en realidad estoy aprendiendo