I suppose you have a text file in something like notepad, and you copy this text to paste into a "editor" of some cms type of webpage, or do you paste it into the html file of the webpage you want it to be displayed?
If it is the first (cms page editor) you can use the editor to format the text again the way you want to display it, or you can see if the editor has a "source" button to paste the text into it.
If you want to paste it into a html file, you need to code the text to the format you want, before copy it.
Remember, a .txt file is plain text, and stripped from all WYSIWYG formatting.
To code your text file, use <p> at the beginning of a paragraph, and </p> at the end of that paragraph, followed immediatly again with <p> for the next paragraph, and so on.