To begin, you'll need to access the Telegram deactivation page. You can do this by going to the Telegram website and clicking on the "Deactivate account" button. From there, you'll be asked to sign in with your phone number and password.
Once you've signed in, you'll be taken to a page that asks you to confirm that you want to deactivate your account. Don't worry, this is just a precaution to make sure you're sure about deleting your account. You'll also be asked to provide a reason for deactivating your account, but this is optional.