The problem with your question here is that it's a very broad subject as there are so many different modifiers that can be used in different ways. The best advice I could give you is to watch LOTS of video tutorial from the net. You can find them at Cgtuts, 3dtotal, cgtalk, youtube and lots of other places. Watch them through and then try and copy what they've done on either the same object or something similar.
Ric