UptimeMonitorBot: Creating and configuring an account at uptimerobot.com
To get you quickly up and running, please execute the following steps.
Create a (free) account at https://uptimerobot.com/
When you are able to log in, at the center of the screen, you should be able to go to your Settings.
Scroll mid-way the page, and you'll find the tab "Alert Contacts". Click on "Add alert contact". You will need the notification URL provided by the bot.
Fill in the required data
Next you need to actually add a monitor. At the left side of the screen, tap or click on the big green button that says "Add New Monitor". Fill in the needed information and select the recently created Telegram notifier.
That was all there is to it! You can add more monitors and select the same notifier for more than 1 site.