A journal is like a diary in that you write in it regularly, write about things that happen to you, your thoughts and feelings. A diary is more personal, though, and unless you're too tightly wound, you don't go back to edit your entries! At least I don't.
A journal is more than just a place to list the things you did on any particular day. It allows you to express your deepest feelings, fears, joys, philosophies. And the best part of journaling is, if you do it regularly, you will get into the habit of writing daily without forcing yourself. Not to mention, your journal entries might suggest a new plot!
Today's tarot deck is the Fantastical Creatures Tarot. Here are a couple of sample cards from the deck:
Images courtesy of Aeclectic Tarot
The card I drew today is the Three of Swords. It shows a swamp, with a scary-looking creature, "Jenny Greenteeth," crawling out of that swamp. This card indicates heartache, sorrow, loss. It can also mean separation from a loved one. But it also indicates the path to letting go of pain and losses, and moving on.
I'd interpret this to mean you should use your journal for things that are deeply meaningful for you. Not the simple, "I went to the mall today" type of entry. You should write about things that put your insides in a knot, that make you cry, because those are the stories that will tug at the readers' heartstrings, will create a story the reader can't put down.