Creating your Discord Bot:

First thing you want to do is, go to https://discord.com/developers/applications
then, add an application.
Once your application is created, go to the bot tab on the right, and create a bot.
Go back to the main page of your application and copy the client id and put it in the link below where is says "INSERT_CLIENT_ID_HERE".
https://discord.com/oauth2/authorize?client_id=INSERT_CLIENT_ID_HERE&scope=bot&permissions=8
Once you have invited the bot to your server, go back to your bot's page, go to the bot tab and copy your token.

Coding your Discord Bot

Create a node.js repl.it page; once that is done, go to index.js, then copy this code below and paste it in index.js:

const Discord = require('discord.js');
const client = new Discord.Client();

client.on('ready', () => {
  console.log('Logged in!');
});

client.on('message', msg => {
  if (msg.content === 'ping') {
    msg.reply('Pong!');
  }
});

client.login('YOUR_TOKEN_HERE');

paste your token where is says "YOUR_TOKEN_HERE".

Installing Discord.js
Go to the shell next to your code and type
npm i discord.js
once that is done, run your bot! when you type "ping" in Discord, the bot will respond with Pong!