You need to ask specific coding questions (preferably with a code snippet and sample skp file which shows the challenge the code is attempting to solve.)
I have a pinned series of wiki lists (in this category) to help with learning Ruby (which is a prerequisite) and how the SketchUp API extends Ruby. (If you need to create an interface for the export, you’ll likely need to use the UI::HtmlDialog class which also requires knowing HTML, CSS and JavaScript.)
You should look at the “official” tutorials on how to set up an extension project …
ADD: I think that TIG has published an open source OBJ exporter over at SketchUcation. A DXF exporter would be very similar as both are plain text file formats. Perhaps also there are some open source XML exporters ?