The WWI docs are available online here. You need to navigate to the site and then will be redirected to invenio, the digital search engine of the archive. Here e.g. is the access to the Heeresgruppen documents. If you clikc, e.g. on Heeresgruppe Mackensen, you'll get the option of "Digitalisat anzeigen", which then takes you to the document (it comes in a pop-up window, so you might want to disable pop-up block when using Firefox or similar).

As for other documents that are not avialable digitally – and as far as I know these WWI files are the only ones available digitally – you'll have to do research in invenio, order them either as guest or via your account and then travel to the appropriate BArch site (Berlin, Koblenz, Freiburg, Ludwigsburg) and can see them there. However, not everything they have is viewable in invenio off-site. For the Nazi party and SS-officer files, you first need to go there and fill out forms concerning privacy and use of your research.

Invenio is also not the most intuitive search engine, so if you are interested in anything other than these WWI docs, I am afraid, the ebst course of action is to visit one of their sites (Berlin for Nazi and GDR stuff, Koblenz for the modern Federal Republic, Freiburg for the military stuff, and Ludwigsburg for the persecution of Nazi war criminals) and work from there.

Alternatively, you can write an email with what you want exactly but ordering stuff will cost you and might take some time since the BArch has a contract with one company, Selke, to handle all their copies and it could take 2-3 weeks to receive stuff.