Decide if this is going to be something you'll do daily or when something interesting happens.
I tried the daily. It became a chore, so I only write in mine when I have something to say. I'm much happier. My first entry was to explain why I was writing it I think.
I tend to go to my journal when I get upset or want a place to vent frustrations but don't need a sympathetic ear. I do it to figure out where I think I am in life and where I want to be too. I write maybe once a month, maybe less. Seems like I never write about the big things, but mostly the muddle in between.
But that's me. I've kept a journal on and off since I was a teen.
You can do whatever you'd please!
Which is the fun of it -- there is so much freedom. You could write about your daily life, about your family and life story, about your frustrations/anxieties or whatnots. You could use it as a happy place when you're feeling negative you go and write or paste things that remind you to stay happy in it. You could treat it like a scrapbook and paste little things you find all over inside and then mark them with little reminders.
Seriously the possibilities are endless.
You could mix all of the above and MORE if you wanted.
Personally I use mine to relieve bad thoughts and feelings that I don't want to share with anyone. My journal is my escape from reality and sometimes I'll use it to write short random blurbs of creative ideas I have in my head, good and bad. It's my little slice of heaven.
I've recently started a creative arts journal which includes drawings, designs, pictures, collages, and writing. I find it more stimulating than just writing and i can put anything I want in there. A cut out from a magazine, a sketch. Its less limited in content than a regular journal.
I'm also working on my masters degree in creative arts therapy so I've done a lot of research on journaling and its effects/uses in therapy. Some different types of journals can be found in this link.
I've been keeping a journal off and on ever since I was in the third grade. I am 13 and I am now on my sixth journal.
I don't write in my journal every single day. There are plenty of days that are plain boring.
I write in my journal practically about everything. You can write everything you want--you have complete freedom. I wrote down my vents, rants, frustrations, hopes, wishes, dreams, etc. I write about my day, my feelings, my crush, etc.