I suggest that you may clone our template at: https://templates.botstar.com/ and see how we design a bot. Also, you may read our document for step-by-step instructions to build a chatbot: https://docs.botstar.com/en/create-simple-bot.html
After designing a bot flow, to publish your chatbot to Messenger, please read our document: https://docs.botstar.com/en/chatbot-publishing.html