So you've got Joomla installed and did so with Sample Data and now your ready to lose all that data and get your own in...
First delete all articles you want and then go into Article Trash under the Content Menu and empty the trash, then you delete all categories, and then sections, then go clean out your menus... Depending on the template you may have a lot of mod_custom content... You'll need to look at each one of those and modify or unpublish/delete those modules.