Most editors request that writers submit work to them using double line spacing, this means that between every line of print there is a blank line. The advantage of this is that there is space to hand write notes on the work if needed.
On a personal note I have also found that editing my own work is dramatically easier if I have printed it out in double or 1.5 spacing for the same reason, more space for comments.
If writers guidelines don’t mention the line spacing required by the publisher I would automatically use 1.5 or double. In doing so it shows knowledge of the industry requirements, but also respect for the editor’s eyes (no writer would want to be known as the one that gave the editor eye strain).
If you’re using Microsoft Word, there are several ways to alter the line spacing. Of these I have always found the one below most straight forward, especially if you’re unused to the process. It involves you typing your work first, highlighting the work you want to alter and the selecting the line spacing that you are after.