Consider how much room you need to write and how neat you need to be with it. Some writers will write on anything and have no problem with a haphazard collection of journals, notebooks and loose pages. Other writers need to have a neat shelf filled with multiple volumes in the same size. Don't try to go against your nature. Stockpile supplies if you must.
Decide how organized you need your journal to be. Do you want to have tabs for character names, locations, dialog, descriptive passages and such or do you just want a free-form set up with no dividers?
Choose a journal that will be convenient to keep with you. If you don't usually carry a bag with you, you may want to opt for a small notebook that can fit into a pocket. If you take a backpack everywhere, you can use a larger book.
Attach a pen to the journal so you never have to look for one.
Turn the journal over and begin writing on the last page if you're left-handed. The binding will be on the right side, and you'll be more comfortable as you write. Or try one that has spirals along the top so you don't have to worry about your "handedness."
Put your name and telephone number in your journal so it can be returned to you if you lose it.
Remember that this is not a personal diary to fill with your thoughts and dreams, nor is it a place to write your "to do" lists.
Write parts of scenes, bits of dialog, a few lines of poetry in your journal as soon as it occurs to you. Most of your best writing should get its start in your journal.
Record conversations you overhear, descriptions of people or events you see and interesting names you can use for characters.
Transcribe passages you love from the books you're reading and quotations that inspire you.
Complete writing exercises such as a daily writing prompt or timed writing.
Jot down any interesting information you come across that you might be able to use in your writing.