On setting up a DNG instance: follow the steps in the  Interactive Installation Guide

Ways to integrate with DNG:

• RRC Reportable API - creating custom reports to be run from DNG or RPE
  

• OSLC RM V2 API - the open standard way to programmatically integrate with DOORS Next Generation, there is an elaborate workshop to get you started here OSLC Workshop article. These programs can be from any language that support HTTP REST calls, e.g. Java, C# etc. Hence these are more flexible to run and can read and update DNG data, as part of a web server, or standalone, but have some limitations set by the current OSLC standard, for example module support.
  

• TRS 2.0 - the OSLC Tracked Resource Set open standard REST API to hook into the stream of low level changes done to resources in the DNG application (and other Jazz apps as well).

• and additionally, there is the so called client extension capability, these are Javascript extension that generally added as widgets you can run from the dashboard, and can be used to view and edit DNG data. It's more tailored to DNG, so has better module support, but is limited to run in a browser/javascript environment.

Getting back to your question what API suits you best, you best ask some questions on how your organization will use DNG and what you want to extract:

• What version of DNG will you be using?
  
• If that's version 6 and up, is your organization going to use configuration management, and possibly components introduced with version 6.0.3?
• What's the scope of things you want to extract? Artifacts, modules, folder structure, tags, reviews etc?
• What's the level of detail you want to extract? Just a snapshot once in a while, or (almost) always have the latest version of the requirements, do you need to have a history as well?
  

With that, have a look at the  OSLC RM v2 and TRS 2.0 API's. If you need a simple snapshot of just requirements, probably the RM v2 API would serve you. If you constantly need the latest and greatest requirements, with configuration management, maybe history, you probably need to tap into the TRS API. Will be quite a learning curve to get your extraction process right though...