I have received a pdf File and now i wanna write something in it....how does this work?

A mere inexpensive way will be converting the entire document to .DOC, making all the changes on it using MS Word and converting it back to .PDF.

Fortunately there are many free online tools to switch our documents between these two formats.

To convert PDF TO DOC :

http://www.pdfonline.com/pdf2word/index.asp

To convert the DOC back to PDF :

http://www.pdfonline.com/convert-pdf/

Hope this helps!

You can't really do that. Unless the pdf was saved as editable document, or you need to have a special software (like Abby FineReader) to convert your pdf into editable MS Word document.

The only way to digitally ' write ' to that pdf is if you own Adobe Writer...

For this, you would need Adobe Professional. Sadly, it's quite expensive.