If you're using an old-fashioned manual (or electric) typewriter then double-spacing is achieved by using 'double return'.
It's far more likely, however, that you're using Word (or a similar program) on your PC. If you've not written the text yet, do the following before you start. (The instructions are for Word but other programs are fairly similar):
Click 'Format'
Select 'Paragraph'
With the 'Indents & Line Spacing' tab selected, change 'Line Spacing' to 'Double'.
Click 'OK'
Now, when you type, the text will auto-wrap at the end of each line as usual but with a blank line between each line of typing.
If you've already typed the text and want to change it to double-spacing, click 'Edit' and 'Select all'. Then follow the instructions above.
If it's a magazine editor or publisher who has stated that double-spaced text is required, remember that you should not leave additional space between paragraphs. Instead, the text on the first line of each paragraph should be slightly indented.
Chris