Did you by chance copy and paste the text from somewhere else? If so, corruption can happen in the pasting part. Best to use "paste special". That will strip all formatting and then you can apply your own, as described by the other posters.
Being a keyboard girl myself, when I do what you are attempting to do, I press Ctrl E to center the heading, type the heading, then "enter" (hard return/carriage return) to get to the next line, then Ctrl L to left justify the remainder (or you could use one of the other styles if you like -- full justification, for example). These shortcuts should work on existing text by placing the cursor anywhere in the text and using the shortcut. The paragraph will be justified as you have told it to do.
Good luck!