The Journal: it should be small, and light weight, so it's easy to carry around. It doesn't even necessarily have to be a journal; it can be in the notes on your ipod, or phone. However most people like having a pen and paper to doodle ideas down. If you are going with the more traditional journal, the pages should all stay inside so you don't lose any. This sounds like an obvious thing, but I've seen many journals with most of their contents on the floor. The moral of the story, make sure the paper in the journal will stay in if it gets tossed around a bit, because if you're carrying it around all the time then it surely will (get tossed around a bit).
The Contents: Like I said before, anything and everything. Some ideas for things to write in it are:
-Quotes. People say funny/insightful/inspiring/passionate things that are worth writing down. I can't tell you how many quotes from people I've spun into scenes. It's hard to remember quotes so if you have your writer's journal handy then it's the perfect place to put them.
-Ideas. Sometimes you'll see something, or hear something and get a brilliant idea for a story that you're likely to forget if you don't write it down. Writers' journals are also helpful to plan out side stories, and other aspects of the plot that you haven't quite perfected yet while you have down time. For example, I plan out family trees for my characters while I'm waiting in the van for my mom to grab some milk from the grocery store.
-Organizing thoughts. Sometimes I have so many "what ifs" floating around my head that are to underdeveloped to add to the plot line, so I jot them down in my writer's journal. I write down the "what if" and start adding other arrow ideas around it until it forms into a reasonable event
that can be added into the story. Here's a picture of a "what if" page from my journal:
As you can see I like arrows.
-Writing scenes. I've been known to scribble down a scenes in my journal. They can be scenes that are currently coming up in your story, or scenes that are completely unplanned. Writers' journals are good for writing your story in a more portable way if you are writing it on the computer. Just write down the last paragraph or so that you wrote on the computer and you're all set up to write. It doesn't really matter if you end up transferring the scene into the actual story or not, because you are practicing your writing skills.
Alright, so that was my "brief" post. I can't stress enough the value of having a writer's journal. Just remember it's there to help you, and make you a better writer. It doesn't have to be neat, notes can be scrawled in the margins, and it can be anything you'd like it to be! As you can see, mine is certainly not neat, but it is an immense help to me, and I've made it my own. I've found this link that talks about writer's journals, and some more ideas for contents of the journal. Be sure to check it out!