How to insert values into an existing PDF on the fly?

Question

 Hi,

 

 

There is a PDF with some fields to accept values from the user(for example: a "bio data" form). My question is that how can I insert the user inputs to the Correct fields of the existing PDF and to generate the filled PDF?

if i using iTextSharp, then how can i choose the co ordinates to print values? 

 

tnx in adv.