Hi!
You do not have to start a journal at the beginning of the year.  You can start it whenever you wish.
You can enter whatever you'd like in your journal,(as long as it's clean!)it can be your daily diary,weekly diary or whenever you want to enter something.It's just not a private diary!! You might get people offering suggestions,comments,thoughts,etc!
It doesn't necessarily have to be all about hair.
I suggest you look and read other member's journals and get a feel of what is entered.