How can someone write something interesting and original if they feel they have nothing new to say? What are some tips for being creative and original while writing fiction or nonfiction books?

As already been stated, read a lot and don't stress to much about being original. Especially read some of the classics and a variety of genres.

But besides that, you might trying some creative writing exercises. Don't worry about making these good they are just to get the imagination going.

For, example rewrite a familiar fairytale ( like Cinderella) and change it's genre (crime thriller)

Write 10 settings, tropes, random items, phrases, ECT on small pieces of paper then blindly pick one of each category and include it in a short story.

People watch, try to observe how they act and think up back stories and motivation.

Write a story with a friend where you write until you get to a cliffhanger then give it to them to write awhile without a plan for how the story goes. This help you be adaptable and not set in just your ideas.