You may not like my answer to this question, but I think you'll find I'm right.
For your first art journal—and for every one you fill after that—the journal itself must be any blank book OR printed book that you like.
It really is up to you.
Well, okay. I can see your dilemma. You may not know what you like yet, and you have many options. There are all the different art journals you can buy—or make. In addition to art journals on the market, you can purchase a handmade journal, you could alter an old book—or you could even create and bind your own book.
With all these choices, how do you decide what is the right art journal for you?
With experience, you'll come to know your preferences. But for your first art journal, I recommend you choose based on these three qualities:
An art journal can be very small, quite large, or somewhere in between. (Its pages can be square or rectangle). It is a matter of preference, but for a first art journal I recommend something in the medium range. Pages that are 5x8" are still a bit small (although it's fine to start there, too). Something in the 8"x10" or 9"x12" range make an ideal size for a first art journal, I think.
Generally, you have two options: spiral coil or book-bound (glued or sewn). People have different preferences on this, so consider this:
With a book bound with coils, you can easily open your book to any two-page spread and both pages will lie down flat on the table. Also, if you like to create separate pages on each side of the coil (what we call the gutter), the coil does separate those two pages. (Although, if you ignore the coil and treat the two pages across the spread as one, that can work just fine.) Also, a coiled book that gets thicker with artwork has some room to expand.
A glued/sewn binding means the two pages in a spread easily one art journal page. The gutter is less visible. Some book-bound journals don't like flat when open when you're working on a spread that is closer to the front or the back (because one side will rest on more pages beneath it than the other).
I do like having less gutter to work over, so I generally choose a glued/sewn binding when I can—but really, it does come down to personal preference. For your first art journal, choose one kind of binding and then for your second art journal (oh yes, you will have a second art journal, you'll see), get a book with the other kind of binding. After that, you'll know for sure. In the meantime, both books will be wonderful!
Paper is the most important element of an art journal. What you ultimately want in your art journal pages is heavier weight that can withstand wet media. There are ways to make any art journal page heavier and water-resistant, but for your first art journal, I recommend getting an art journal manufactured for mixed media. Then you can be assured it can withstand all the juicy goodness you're going to put into it!
And that's it. It's usually easier to purchase your first art journal and you'll find plenty of blank books made for mixed media art journals at almost any art supply store. Just think about those three qualities when you choose your first art journal. Eventually, if you're like me you may end up keeping several different kinds of art journals for different purposes, each a different size, bindings and paper weight. But for your first art journal?
And for all the others in your future...
Choose what you like.