If you’re a student who’s interested in history, you can always pursue it as a degree. That’ll give you the training and access to the kinds of resources that will let you do the research you want. If you’re not a student or would just rather pursue a non-academic path, I’d recommend heading to a nearby university library. You may not be able to check books out, but you can still read books while you’re there. The reference librarians would be able to point you toward books covering your specific interests. World War I is a big topic (historian Christopher Clark said there’s something like 25,000 books and articles in who knows how many languages!), so that should keep you busy for a while. One other small point, and I think this is true of “diplomatic and military history” especially, start with older books. They’re going to be more narratively structured and easier to understand. Then work your way up to newer/more sophisticated stuff. Academic writing can be infuriatingly unclear or obtuse sometimes, so it’s best to go in with a strong basic knowledge of events. Also, some journals (which again, are best accessed via university libraries) publish roundtable discussions about specific topics (though I don’t know if there are any on World War I). These are good for getting multiple perspectives on a given topic.

Best of luck!

The best thing that I can recommend is get your hands on a good, up to date textbook. You don't need to buy it, you can just go into a library and interlibrary loan it. Even your local libraries will (usually) do this.

For your case, I have a copy of Ian Beckett's The Great War. The reason why a textbook is useful is it's introductory, it will incorporate new directions in scholarship, and, most importantly, it will be well cited. Beckett's book has lengthy footnotes after every chapter.

Now, maybe you don't read an entire dry textbook, but you find the chapters on the aspects of the subject that interest you and then write down titles related to what you want to learn about. With a little patience, you'll be able to access just about any book via an interlibrary loan system at your local library. I see lots of people on this sub who look for internet or youtube resources because they're easy to access, but books are almost always your best bet.

This method can be used for any number of subjects. If it's a subject of historical inquiry, there is an introductory textbook. Some might be more technical than others, but they are designed to give you an overview of the subject. As you expand your interests, it's good to familiarize yourself with the publishing houses of Routledge and Willey Blackwell. They both produce high quality books and they can be used as a roadmap.