The question you are asking is too broad for an accurate answer. I think you would need to try an provide an example starting point and a sample of the type of illustration you would like to have as a final product. Very likely your goal could be achieved with the native Photoshop tools.
There are several photograph to illustration plugins which help facilitate the general process about which you may be inquiring.
Snap Art may be the best all around package…
<
http://alienskin.com/snapart/snapart_examples.aspx>
Flaming Pear has India Ink…
<
http://www.flamingpear.com/indiaink.html>
Andromeda has Screens, Cutline and Etchtone…
<
http://www.andromeda.com/main/screens.php>
Akvis has Sketch…
<
http://akvis.com/en/sketch/index.php>
