How do I completely delete wordpress and start again?

Do you want to delete all files and the entire database too?

Just ftp your server, delete the folders which have all the wordpress stuff in it:
wp-admin
wp-content
wp-includes

Then re install WordPress.

then start over

withing the folders you could eliminate individual folders as well to delete plugins, images, but to wipe it out and start over just delete the above listed folders and all of the folders content

You’ll need to drop the tables (via phpmyadmin)that WordPress is installed to, or your just going to run into some major issues.

I want to delete the entire database. Everything. How do I drop the tables?

You’ll need to go into phpmyadin, select your table and hit ‘Drop’. Just from a quick google search I was able to find this