open , read and extract some data from a CVS File and then write the extracted data in a new CVS or TxT file

Hello everyone, 

I have to open ,read , extract data and modify its format   from many cvs files. Then I have to write or copy the extracted data into a new txt/cvi File.

I've read,opened and extracted some data in a Buffer but I couldn't write them in the txt file created at the beginning .So that I need your help.

ps: I didn't really get how the function "WriteFile" does work 

thank you in advance