Unlike using a typewriter, authoring documents on a computer is not typing in a “what you see, is what you get” (WYSIWYG) manner, as it’s freqently suggested by desktop publishing software producers; it’s just an approximation.
When using a typewriter, you see everything typed on the page, as you type it, and know exactly what the end result will look like (the same as it looks as you’re typing the document). But, when you use a computer, you’re seeing an “approximation” of what the end result may look like. This is because the screen is a very different medium than paper (it has an inferior resolution, different proportions, and different colour responses). Although the system will try to render the screen in a similar manner, it’s unusual to get a precise preview of how a page will print. Even viewing the same document on different screens, produces different results.
To a large degree, the difference is due to the differing mediums, but there’s also a lack of information given to the computer about how the printer will respond (they’re all different). The monitor and printer drivers aren’t usually brimming with specific information, leaving the computer to make guesses and assumptions.
The rendition of the fonts (shape, style, weight, size, resolution, etc.).
The spacing between characters, words, and lines.
The positioning of content.
Desktop publishing (generally) handles margins “paragraph by paragraph,” rather than over the entire page.
Margins and borders.
For these reasons, and more, you author documents in a different manner than when you’re typewriting.
Test print any fonts that you choose, to check that they’re readable, and that they print in a sensible manner.
Where possible, instead of using fonts and margins, etc., to change the look of special parts of a document, use “styles” to change the style of special portions of the document. You can (then) change all the styles from a style editor, without having to re-edit the entire document, should you need to change the look of the document.
Do not use blank spaces and carriage returns to format a document. Use tabs, and the rulers to position margins, text, text boxes, images, etc.
Test print any colours that you use.
Test print where your margins and borders are (so your document doesn’t try and use parts of the paper that printer cannot print on).
As well as all of that, desk top publishing can do much better typography than a common typewriter (proper quotes; character, word and line spacing; etc.), so it’s worth doing things properly where you can (it looks a lot better).