To go back to zero

Well, since you aren’t really sure what you changed, I suggest deleting the directory you installed WordPress in, and re-installing from there. If you installed it through your admin panel (cPanel, Korax Admin, etc.) I suggest you look there at your WordPress Installations – you should be able to uninstall from the same area you installed from (unless you installed it manually).

If you have to remove the files manually, I also suggest going into your phpMyAdmin and removing the database that WordPress created. To find it, go into the wp-config.php file in the WordPress root folder, you will see the database name (and credentials if you don’t know them) to get into phpMyAdmin.

As far as your request about themes, you can definitely have as many as you like, you need to just activate whichever theme you wish to use.

Removing the author, date, and the blogstuff on a post: You need to have some coding expertise to do this. You have to edit the “loop” that creates the blogroll on that page. You will find it in index.php or front-page.php (depends on the theme). Some themes may have theme specific settings to hide such things.

My best advice is to read through the codex thoroughly before trying to do anything with the php files, and the more research the better!