important : Fill Excel File using C#

Question

Hello everybody. I m trying to fill excel file from c# more than one month
I know here is experienced people who can give me some useful advices.
So... There is .xls international Form in it is lots of columns rows and texts. I want to program c# app which can insert values in current place. for example insert 239 USD in 27, K (this is a address of excel forms cell)
Does it possible?
What is your advice?
If it easier way to use html or something another?
i need step by step tutorials or book or something
It is very very important for me
Thanks