Ok, there are plently of tutorials around, you just have to google or look on youtube.
But heres a way to do it if you have a white background, it also helps if you have hair to extract.
Duplicate layer, then fill original layer with colour of your choice, choose a colour most different to your original image.
Now select the top (duplicated) layer,
Go to > layers>Layer mask>reveal all.
Then go to Image>Apply image (make sure to check 'invert image'
Now go to your channels box. Make the bottom mask layer visable, then uncheck the RGB layer at the top. You should have an inverted B&W image.
Now use the dodge tool (set to highlights) to lighten the light areas, and the burn tool (set to shadows) to darken the dark areas. Avoid going near the extremities of the hair.
After you have done this, make the RGB layer visable again and uncheck the mask layer.
Select the mask layer (dont make it visable, just select it) and go to> Image>Adjustments>Levels.
In the Top half of the Levels panel (Input Levels) move the left marker to the right to remove any haloing. What you do at this stage depends on what you did on previous steps, so you might have to do the opposite i.e move the right marker to the left. This basically allows you to fine tune your layer mask.
Once you've done this you can put in any background you want. Or Work on your background first, then import the istock photo and follow the steps as directed.
With a bit of practice, it is the best way to extract hair that i know of, good luck!
Adey