A journal is there to remind you of your future goals and ideas and can help bring you back from the outer reaches of a dark space. That's usually how I think of it when I feel down and drifting: that I am aimlessly meandering in a vast blackness dotted with the distant twinkling of stars that sometimes reflects my state of mind. My journal becomes my spacesuit, linking me back to civilization and alternate mind sets, all I need to do is focus and use it to help me find my path.
If you already journal, then I'm sure you have have already found solace reflecting on your old entries. It's awesome to track your progress throughout the year, laughing sometimes, sighing other times maybe even cringing occasionally. If you don't journal yet, then get out there and pick one up to get started.
A few months ago I outlined my 7x7 method(link) of attaining goals. This can be a great way to start a journal if you aren't sure of what else you might want to write down. I always find my journal to sound less like a diary and more of a place to organize my thoughts and ideas. I write less of a dear diary style and more often in bullet points and mind maps.
The best part about your journals is that it doesn't matter how you organize it as long as it's something at works for you. Fill it up as you please, in a way that makes you happy. Keep it somewhere handy at the end of the day. For me that's under my pillow so I always know where it is before I go to sleep. And then it's there first thing in the morning as I'm getting ready for the next day.
Remember, this is for you. Find what feels right and pursue that method. You might want to create sketches over breakfast of your dreams. Awesome do that. However you get your journal to work for you, just give it a shot and see how an ink filled book just might change your life.