How to create 404 page

i have error.txt on directory of Content. But how can i visible the page? Can i solve page not found problem in config.php? if i can, how?

The textfile should be in content/error/ rather than directly in the content folder. And then you should see it if you cause a 404 error (by trying to go to a page that does not exist). To give it a unique layout etc, you will of course need an error.php file in the templates folder, and a corresponding blue print to set anything custom you would like on that page.