It all depends on what you are trying to do exactly as in most cases you don't have to do anything special as such.
Line-height is the space between lines of text and you just set them to suit the design and the size of the text. I usually just set a "unitless" line-height on the body of 1.4 and that will cascade down into the page as required. A unitless line-height (as opposed to em or px) passes a scaling factor down to the children and thus maintains the 1.4 relationship for all sizes of text in the document (there may be browser variances but in most cases is fine).
If you set a px line-height then you would usually base it on the size of the font and then add extra so that you have room around the text. e.g. if you have 14px high text you might want a 20px line-height.
If you are using em then you would usually use something between 1.3em and 1.5em for good readability.
Generally between 1.3 or 1.5 times the font size is a fair guess at a reasonable line height.