Assuming you're using something like Word or similar on the top line of your screen is a menu selection reading
File Edit View etc.
Select view and scroll down to Zoom. Choose 100% - That way you will see the text at the size it will be when printed out, although you may not be able to see the whole page depending on the size of your screen.
If you want to increase the size of the font highlight all the text and change the size in the toolbar at the top of the screen. It'll look like a little window with a number in it (10, 11, 12 are defaults in most of the applications) There's an arrow pointing down immediately on the left. Click and hold and then scroll down to select the font size you want.