Selenium using Python - Geckodriver executable needs to be in PATH

I am going over Sweigart's Automate the Boring Stuff with Python text. I'm using IDLE and already installed the Selenium module and the Firefox browser.

Whenever I tried to run the webdriver function, I get this:

from selenium import webdriver
browser = webdriver.Firefox()

Exception:

Exception ignored in: >
Traceback (most recent call last):
  File "C:\Python\Python35\lib\site-packages\selenium\webdriver\common\service.py", line 163, in __del__
    self.stop()
  File "C:\Python\Python35\lib\site-packages\selenium\webdriver\common\service.py", line 135, in stop
    if self.process is None:
AttributeError: 'Service' object has no attribute 'process'
Exception ignored in: >
Traceback (most recent call last):
  File "C:\Python\Python35\lib\site-packages\selenium\webdriver\common\service.py", line 163, in __del__
    self.stop()
  File "C:\Python\Python35\lib\site-packages\selenium\webdriver\common\service.py", line 135, in stop
    if self.process is None:
AttributeError: 'Service' object has no attribute 'process'
Traceback (most recent call last):
  File "C:\Python\Python35\lib\site-packages\selenium\webdriver\common\service.py", line 64, in start
    stdout=self.log_file, stderr=self.log_file)
  File "C:\Python\Python35\lib\subprocess.py", line 947, in __init__
    restore_signals, start_new_session)
  File "C:\Python\Python35\lib\subprocess.py", line 1224, in _execute_child
    startupinfo)
FileNotFoundError: [WinError 2] The system cannot find the file specified

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "", line 1, in 
    browser = webdriver.Firefox()
  File "C:\Python\Python35\lib\site-packages\selenium\webdriver\firefox\webdriver.py", line 135, in __init__
    self.service.start()
  File "C:\Python\Python35\lib\site-packages\selenium\webdriver\common\service.py", line 71, in start
    os.path.basename(self.path), self.start_error_message)
selenium.common.exceptions.WebDriverException: Message: 'geckodriver' executable needs to be in PATH.

I think I need to set the path for geckodriver, but I am not sure how, so how would I do this?