Try this:
http://www.webreference.com/html/tutorial18/
Absolute positioning can be valuable, but it is very tricky in most situations. For example, if your design is centered, you obviously cannot use absolute positioning in the horizontal, since the design will shift according to resolution (or rather, viewable browser width). The other big snag is that you cannot prevent Firefox (and other) users increasing font sizes, although there are ways to limit the chaos that can cause your layout.
Usually it is best to go with (the default) relative positioning and "graduate" to absolute positioning once you are stronger on CSS and only then, if you must.