To display text directly on the screen (name of level for instance), send this command: msg player (teletype 9524) The number here is just an example but it refers to the string with your own text.
For a computer or note in the level, you add a story object, and in the text ID field you enter the number from the cres file where you have edited your text.
Like mentioned, it's good to make a copy so nothing gets messed up by accident.