There will always be more that you don't know, and experience will tell you what details are important to remember and what are less so. I'm a hopeful specialist in U.S. foreign policy, but I can't remember the exact day the Marshall Plan is announced or all the blow by blow details of UN negotiations. That is all information that I can check fairly easily with a jaunt through a general secondary or tertiary source.

To prepare yourself for studying history at the university level, what I recommend is that you approach history with a few questions in mind:

What is the argument of the author that I'm reading or the lecturer that I'm listening to? Are there any assumptions that undergird their argument?

Who are the subjects that we're discussing? If we're talking about the elite and the powerful, what does their perspective tell us? What would talking about ordinary people, or minorities, or other demographics, tell us instead?

What does learning about this specific thing fit into a broader framework of understanding? There will always be nitty gritty details to learn. But tying that to a broader concept or narrative can really help you categorize what is important and what is less so for you.

There are some books that can help you learn to think like a historian. The Landscape of History by John Lewis Gaddis is a short and very readable book about what historians do and why it's important. Parts of The Historian's Craft by Marc Bloch are also good treatises on what historians do (an older book, but worth the read it if you're interested in this stuff. Bloch himself was writing this book in the 1940s when France was occupied by Germany, and he was executed by the Gestapo before he fully completed it.)

And lastly, if you're really just concerned about not knowing enough information, don't be afraid to just go to your library's catalog and look up books or journal articles in your subject. But a good history class should give you the context for understanding what you're being taught.

If you never had one, get a book on academic writing and style. For example, The Manual for Writers of Research Papers, Theses, and Dissertations: Chicago Style for Students and Researchers is, in my opinion, a very useful reference work for language and style in English (esp. if you actually have to write in Chicago Style at your college/university *laughs*), whereas The Craft of Research is more focused on argumentative writing (of which the former also touches on, though) at all levels, omitting the whole discussion of proper grammar and style.

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I also read a few introductions on effectively the history of history as an academic discipline - there should be something out there in English along the lines of "introduction to history": books which explicitly give an overview over the various schools and styles of thought historians practiced in the past 150-or-so years, such as Historicism, Social History, Cultural History, the French Annales School, Postmodern and Poststructuralist influences of the recent decades and rise of Cultural History, Gender History, etcetc.; these kinds of introductions, or overviews, also tend to touch on landmark works and authors of the respective approaches. (Unfortunately, most books of this I read were in German, so I doubt recommending those is helpful; for example, Lutz Raphael's Geschichtswissenschaft im Zeitalter der Extreme was one of them, but apparently that's not translated to English, so...)

I quite enjoyed John Arnold's What is Medieval History? and the similar books by Peter Burke, What is Cultural History? and History and Social Theory. Arnold's book is obviously significantly more useful if you actually study medieval history; however, all three books are easily digested and written in a quite entertaining style.

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Lastly, no matter what you study, don't limit yourself to only reading books from within your discipline. Especially classic works of sociology and anthropology provide very useful ideas for historians to think about their subject of research - and most good scholars of these other disciplines are acutely aware of history. (the borders between disciplines are more artificial than real anyway)

But branching out can wait a few more years, should you endure :)