Find a notebook or journal of some sort. Don't drop $100 on your first book, save your money for something else. Just buy a couple of composition book, spiral notebooks, a 3-ring binder, a journal, whatever. Decorate it if you want to, with whatever feels right for you, whether that be nature images, symbols, deities, etc. This is YOU book, do what you like with it. I personally had (and still maintain) two separate books.
I use one for recording things like my personal thoughts, tarot and rune readings, rough drafts of things, ritual and spell records, etc., and the other for correspondences, finished spells that were successful, my beliefs, etc. If you want to do things that way as well, that's fine, but many people also just keep it all in one book.
What I suggest doing, right now, is writing down exactly what you believe. I think I mentioned this in the past either on here or in a video... Write down how you see the God and Goddess, your thoughts about karma, death, the after life, morals for spells, etc. Anything you can think of that would be considered you belief about your spiritual path. I would also suggest writing down WHY you've decided to study Witchcraft and (if it applies) Paganism. I do this about twice a year still and I would recommend you do the same. It helps you see where you were and where you are now, as well as help you figure out what beliefs are probably here to stay. For a while, you may read things in a book and think, “Oh that makes sense!” and you start to believe it as well, so you write it down. But, two years later that belief may be different after you've formed your own opinion. Some people may totally know exactly what they believe right off, and very little if anything may change throughout their life. However, I think that most people undergo too many changes in life to not see a few things changes.
When you've written down your beliefs, it's wide open. Research all you can. Take notes, and when you find something that you think is important, copy it into your book. I would recommend having things such as: Correspondences of things like the elements, days of the week, moon phases, sun signs, moon signs, herbs and stones (if you'll be using them, and I suggest meditating with your stones to see what they're useful for for you, instead of just copying from a book), colors (if you want, though I suggest making up your own based on what you feel represents what); Sabbats if you observe them; Deities you've worked with or want to work with; successful spells; rituals; how to cast a Circle; calling quarters; information about any divination tools you use (such as tarot cards that are most meaningful to you or all of them, how your pendulum moves for answers, tarot spreads, runes, palmistry guides, etc.); information about the elements (particularly your own impressions), etc. Basically, anything that is useful to you for your practice. These are just suggestions to get you started. I personally have a page about the Circle, and rituals, and calling quarters, but I only use them every now and then and I've considered removing them. I also have a new section for DIY things such as cleaning products because I tend to weave in spells with my cleaning. And I have a recipe tab, for things I make around Sabbats and Esbats...
Do not try to cram a lifetime of information into a few weeks of study. It won't happen. Take your time learning as well as adding to your book. You have your whole life to finish it, if you remain on this path. I fully understand the desire to put as much information as possible into your book as quickly as possible, but it really isn't necessary. This isn't a race, it's a learning experience that takes a long time.
As for fancy, expensive books... I suggest taking your time with that, as well. Many of you may know that I received one as a Mother's Day gift from my husband this year. I still haven't written in it, but that's mostly because I do not have ink or a dip pen to practice writing with. I'm in no hurry, though, my current 3-ring binder is expanding all the time.