Find out my own user ID for sending a message with Telegram API

You could use Json Dump Bot.

The relevant section of the output will be similar to:

{
  "message": {
    "from": {
      "id": WHAT_YOU_ARE_LOOKING_FOR,
      "is_bot": false,
      "first_name": "Paolo",
      "language_code": "it"
    }
  }
}

or: https://t.me/userinfobot - just forward message to get his id

Send a message to your bot then retrieve the update with:

https://api.telegram.org/bot<BOTID>/getUpdates

In the json string returned you will find you chat_id/user_id.
In single chat with the bot chat_id is equal to user_id.
Now you can send your message with:

https://api.telegram.org/bot<BOTID>/sendMessage?chat_id="yourchatid"&text="Hello!"