System Preferences->Users & Groups->Select the user, and click the minus at the bottom of the window.
You'll need to click the lock to make changes.
Note, you should back up this data. There are scads of people here asking for how to restore files down the road and it's not always easy if enough time passes.
Just go to System Preferences > Users & Groups and choose the account you want to delete. Click the lock to open and then authenticate, then use the minus to delete.
But you can't delete the original user account you set up when you first booted in Setup Assistant. Sometimes it's easier to start over and do everything right in Setup Assistant.