Hi Amy & welcome:
What type of Kindle do you have? I've got a K1 which I think has a different method from the K2s or DXs.
you basically need to get to your menu to 'manage content', then there will be some way to select a book/sample, and then bring up a menu that will let you choose delete content
One of the software updates (I think it was sometime around the announcement of the K2) gives you another option for deleting content on the K1. You no longer need to go to "Manage Content" - instead move the cursor to the line with the item you want to delete and hit the backspace key on the keyboard. You'll get the prompt for deleting it. Much easier and faster - really should have been there from the beginning. Now for folders...