I agree with Richard: if this is a new app firing every few minutes, then you will need permanent storage outside the app - but I'd suggest that a Database might be better than a disk file, as they are better organised and require less "thinking about" when it comes to adding, removing, or changing data.
Have a look here:
Walkthrough: Saving Data to a Database (Single Table)[
^] it's not that complicated when you get your head round it, and it's stuff that it's well worth knowing!