this is where tutroials come into good play, and thank goodness for the new tut forums.,
what i hope you are meaning is, taking out the chracter from an existing background.,(which is also known as extraction).
luckly for you, Jade has the right tutorial for the job(i highly approve of this one)
[photoshop] Extraction - easy/hard
FYI, the polygonal lasso tool, is very simple, it may look like its gonna to straight egde grossness but when you make a selection out of it , it smooths out and curves to your image making it perfect.