If you're allowed to use your textbook, highlight the important bits when you do your reading. You can also use different colors, like blue for headings, pink for examples, green for proofs, and yellow for equations & important constants (science example; would be different for history, etc.). Also making notes in the margins can be helpful for if you wanted to keep track of additional information. Make sure you know the layout of your book, and use the index to make searching for information quicker.
If you're only allowed to use your notes, make those notes as easy to navigate as possible. I've been using an adapted version of the Cornell system for note-taking--there's plenty of stuff online about it if you want to check it out. It makes it a lot easier to use my notes as a study tool instead of just a record of the lecture. A lot of my professors have used Powerpoint for lecture, and then they make those notes available ahead of class. Some people like to have those on hand so that they already have a record of the lecture and can pay attention better. Me, if I have the notes ahead of time, I find that I kind of 'zone out' during lecture until it's nearly over, so instead I have taken to forcing myself to take notes.
Sometimes they'll let you take an index card or a sheet of notes with you (they let us for my polisci class because of the structure of the exam). Make sure you talk to your professor about what will be provided on the test for you (e.g., formulas, constants, periodic table) so that you don't waste valuable space writing down a piece of information your prof will give you in the exam.
Also, if you are allowed, bring post-its or sticky tabs into the exam--helpful for marking your place when you are flipping through the book and looking for information to support your arguments. Plus if you write quotes or information on the post-its, you can arrange them on your desk to literally build an argument and help with making seamless transitions between paragraphs (I always get docked points because of that!).
Chances are, if it's open-book, they will be expecting an essay (or several), because they want you to be able to use/analyse the information that you're given in your text/notes. Anticipate potential questions that could be asked of you (usually the prof will say something like "this is very important" or whatever when the subject is likely to be on the exam), and try and come up with a handful of arguments. Also, skim all the essay questions first, especially if you can choose one of several questions to answer. Pick out an easy one to do first to build your confidence, and then move on to more difficult questions.
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