is there who can help me how to use mailgun?
That depends on how you plan to use it. Do you have an existing script? It probably has e-mail settings somewhere where you can configure a custom SMTP server, which you can use to configure Mailgun. You can find the required settings in Mailgun’s documentation. If you’re building the software yourself, using the Mailgun API is probably easier (you can find the documentation on their website).
then what should I do next?
then what should I do next?
Ask Mailgun, you’re trying to use their service with your website. I can only help you in using our hosting, which is pretty irrelevant in this case.