OK - now that we have more info, it's easier to answer your question.
This is one of the most common techniques in Photoshop (and I'm assuming you're referring to changing the background of your image in Photoshop, so I've moved this to the
PS forum). There are literally dozens of ways to do it. There is a BioRUST tutorial that deals with one technique here:
BioRUST.com :: Tutorials >> Extracting Hair
For other methods, have a look here:
photoshop tutorial remove background - Google Search
If you could post the image you're working with, it might be easier to suggest the best method.