How can I do this?

I will make a if function but I don't know it to do. When a Section is selected than should run this if and write in a file a text. When the Section in the installer is not selected than should not write in a file. That is logical. I will it not write this in the section because than is the text in the falsh order.

I'm not sure I exactly understand what you want to do. You want to write to a file but not from a section, because then it would be in the wrong order in the file.
Maybe this helps: use .onInstSuccess function to do what you want. Use SectionGetFlags to retrieve information about selected sections.

-Hendri.