Hello!
This tutorial will demonstrate how to create a simple Twitter bot using the Tweepy library in Python.
This tutorial will be separated into four parts:

Prerequisites

Setting up a Twitter Developer Account

To create a Twitter bot you must apply for a Twitter Developer Account detailed in this link.
You must describe the use-cases that you intend to satisfy using their API. This application process can take anywhere from an hour to a day, so be patient!
Once accepted you will get an email detailing how to access the developer portal. Login to the developer portal and follow the steps to create a Twitter app.
Make sure your app has Read + Write + Read and post direct messages permissions by going to this screen:

Once it is created and the permissions are set, hit the Key 🗝 icon next to your app, and you should see this screen:

Generate and take note of the API key/secret and the access token/secret. I recommend putting them in a text document, as you cannot access them again once they are generated.
Next we will go over creating the application.

Creating the Application using Python

We will be creating a Twitter bot that simply tweets Hello world!
The file structure by the end of this tutorial will look like:

├───twitterbot
  └───.env             # File that contains our keys
  └───main.py          # Our main application
  └───requirements.txt # The packages we will be using

To start out, create an empty directory using:

$ sudo mkdir twitterbot

And cd into the directory using:

$ cd twitterbot

Create two files called main.py and requirements.txt.
In requirements.txt, insert the line:

tweepy==3.10.0
python-dotenv==0.17.1

Then, to install the required packages, run:

$ pip install -r requirements.txt

tweepy is the library that utilizes the Twitter API, and python-dotenv is the library that allows environment variables to be utilized in your program.
Second, we will go over where to put the keys/tokens.
Create a file called .env and paste the following with the keys you generated in the earlier steps:

consumer_key=XXX        # Found in the API/Secret section
consumer_secret=XXX     # Found in the API/Secret section
access_token=XXX        # Found in the Access Token/Secret section
access_token_secret=XXX # Found in the Access Token/Secret section

It is a good practice to utilize environment variables when dealing with sensitive credentials like this. Never put your credentials directly in your code.
Next we will go over how to utilize the credentials in our code. In our main.py file, add the following:

import os

import tweepy
from dotenv import load_dotenv

# Loads the .env file for the credentials
load_dotenv()

# Credentials set in the .env file
consumer_key = os.environ.get('consumer_key')
consumer_secret = os.environ.get('consumer_secret')
access_token = os.environ.get('access_token')
access_token_secret = os.environ.get('access_token_secret')

We will now implement the tweepy library to use these credentials and authenticate with Twitter:

auth = tweepy.OAuthHandler(consumer_key, consumer_secret)
auth.set_access_token(access_token, access_token_secret)

# Create API object
api = tweepy.API(auth)

# Checks if the credentials entered are correct
def authenticate():
    if not api.verify_credentials():
        print('Authentication: FAILED')
        return False
    else:
        print('Authentication: OK')
        return True

Next, we will implement a function that handles the tweet itself as well as a main function:

def tweet():
    # If authenticate returns true, execute the following
    if authenticate():
        # Calls the API to tweet the following
        api.update_status('Hello World!')
        print('Tweet has been sent!')
    # If authenticate returns false, print the following
    else:
        print('Tweet has not been sent.')

if __name__ == "__main__":
    tweet()

Running the Application

Now that all of our code has been implemented, you can run the program using python main.py and you should see the following:

Authentication: OK
Tweet has been sent!

And you should see that your tweet has been sent:

Congratulations! You have successfully created a Twitter bot using Python!
If you need to view the files we created for any reason, check it out on GitHub: