Fix Python – Selenium using Python – Geckodriver executable needs to be in PATH

I’m new to programming and started with Python about two months ago and 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()