Like always its probably been answered and I couldn't find it.
1)Login in to you admin section
2)Go to you catalogs first page where all the main category folders are listed and delete them and the products in them
3)Go to your favorite sql program such as phpMyAdmin and run this sql statement.
TRUNCATE TABLE products; TRUNCATE TABLE categories;Thats All!!
The first category and the first product you add will be #1