How to have my own website for selling SMS?

You can white-label your domain/sub-domain and customize your website as per the themes available.

You can learn how to white-label your website from the link mentioned below:

https://msg91.com/help/how-do-i-white-label-my-website-how-to-point-cname-for-the-domain-or-subdomain

Also, you can learn to customize your website with the help of the below-mentioned article:

https://msg91.com/help/how-to-create-and-design-website-for-selling-sms

For a video tutorial refer to the link: https://www.youtube.com/watch?v=EPrDYCyBX0g