A sketchbook or journal is exactly what you put into it. It's a place to explore, or a record of your life or part of your life. It can be a tool or an end in and of itself. That being said, check out "Artist's Journal Workshop creating your life in words and pictures" by Cathy Johnson from your local library if you can. It's just full of ideas and has you thinking about what you want from your sketchbook or journal. Having an idea of where you want to go is helpful as long as you don't let it lock you into something or something that keeps you from creating.
A sketchbook/journal is a very personal thing. It can also be very private. I know that the people who share their work here on the forum are not showing us everything they do. Some pages are private/personal and some just aren't worth the time it takes to photograph or scan them. I know that the case with me.
For me, I live in a very small space. I don't have a studio or even a table that I can do large pieces of art. So my sketchbooks (yes, more than one
) are what I do for art. The thing is, even tho' it's just a sketchbook or journal, the more I do the better I get. But, it's not required to be a masterpiece so I can relax and just do art. And, if I make a mistake, I can just turn the page.
The other is, as everyone else has mentioned, carry your book and mark making supplies with you. For some that is simply a pen or pencil. For others, it's everything.
Usually it's somewhere in between. I generally carry, a couple of graphite pencils, sharpener and eraser, a handful of colored pencils and another handful of watercolor pencils and a waterbrush. I have a large "back bag" that carries all of that, my camera and my other purse essentials. Sometimes I carry pens too. Many times, the colors stay in the bag and I'll just do a quick pencil drawing, some notes and take a picture. Other times, I'll get everything out. Depends on the location and circumstances. When I do a quick sketch and picture, sometimes I'll "finish" it later and sometimes I move on.
A lot of these answers depend on what you want to do in your art journal. There are all different kinds of media that can be used. Or collage elements or even combinations of things. Or doing multiple layers of media. Try things, see what you like. Share if you'd like to or not. Mostly, have fun.