You have a couple of options here:
To start over with the same site address (URL)
If you want a site but don't want any of the posts and pages you have now, you can delete all of your existing content.
- Go to the My Sites tab
- Open the Blog Posts and Pages sections
- Tap the "Trash" link below each item to remove it
You can also empty your site's content in a web browser with this guide:
Too much content to delete it all on your own? Please contact our support team at help@wordpress.com from the email attached to your account for more help! We ask you to go there rather than here for security reasons.
To start over with a new site address (URL)
To start over on a brand new site, at a new address:
- Go to the My Sites tab
- Select Switch Site at the top of the screen
- Tap the plus (+) icon in the top right
- Select Create WordPress.com site
- Choose your new site's name and address
Hope this helps. If you have any other questions, please feel free to let us know :)