The first step is to get the API token for you bot. See this guide on how to do this.
Next, navigate the following URL, replacing YOUR_BOT_API_TOKEN with the token retrieved by following the post above:
https://api.telegram.org/botYOUR_BOT_API_TOKEN/getUpdates
Assuming your bot API token is:
678918734:BCEwt6h1je_l9FBN684BNM-Eo4ja38wh740
An example of the full URL will be:
https://api.telegram.org/bot678918734:BCEwt6h1je_l9FBN684BNM-Eo4ja38wh740/getUpdates
Next, get the user who's chat ID you'd like to obtain, to send a message to your bot. Under the result object you'll have a list of numbers, click on the little arrow next to the number on the bottom of the list, and navigate to message > chat. You will see the chat id just above the first_name field. See the screenshot below:
!["chatid"]("__GHOST_URL__/content/images/2018/07/chatid.png")