I just write what I have in mind. Maybe something that disturbs me, or what I have planned but failed to do, some regrets. I think it's important to leave all negative you have on a paper. And then, I write what was exciting about today! And hopeful thoughts. This encourages me.
I like starting with the exercise of writing for five minutes whatever goes through your mind, without getting stocked in any specific thought.
I write a short journal every night. It is only pne or two lines long and is a memory for next year. Above each entry is the entry from the previous year. This way I can see changes in my habits, happiness and lifestyle. I can remember the bit stuff as well as the little stuff. For example how tired I am from work compared to how wonderful my 30th birthday was.
I created a special place just for my writing. I have certain things that inspire me—-a unicorn statue, incense, inspiration images and a candle. I bought a special gel pen just for journal magic. Create your own scared space… Good luck!