An art journal is a place that you can doodle, draw, paint, collect images, stick in ideas - little things you see that you like such as postcards or stickers, bookmarks, flyers from art galleries... whatever you want - put it in there. An art journal can be where you collect images that you find inspiring, may one day want to paint like, may be doodles that you develop into art, it's somewhere that is just for you. No one else need look at it if you don't want them to.
So where do you start? Start with a blank journal, or some blank pages that you can put into a folder, start with an old book and paint over the pages. It doesn't matter - really - it is totally your call.
I use blank art journals A4 size that I got from our local office suppliers, acrylic painting pads and watercolour painting pads. I use the acrylic painting pads for when I want to play around and develop an idea, as the paper is very tolerant of being painted over and over again. Ordinary paper tends to tear if you use too much paint or too much water.
Water colour pads are great for light watercolours - the cheaper ones are not very tolerant of too much enthusiasm in brush technique, but it really depends on your budget, how much you wish to spend to start out with.