To do this, choose Paragraph from the Format menu---a dialog box will appear shortly on your screen. The paragraph format box includes spacing options for above, below, and leading. The latter is the distance between the lowest letter on one line, and the highest letter on the next.
Line spacing is then the leading, plus the default font size. For example, if your default font was 12pt, and you wanted to double space the document you would want the leading to be 14pt (it is 2pt by default).