I basically write about today's thoughts and feelings. About about off today's personal triumphs and any challenges and nite I could have done things differently
You follow a set structure of questions. Eg. What are the three things I'm most grateful for today? What was the funniest thing that happened today? What can I learn from today? And a few more general lines in the beginning if you want to. I recommend keeping it short on the beginning in order for it to be an easy habit to adopt
Think of a deep topic, something that you can really get some perspective on in your life. Then plan out your entry. Just be honest with yourself and write meaningful things.
I keep my journal right by my bed and I have a pen next to it that I never move. I make a point of opening it before bed every night, even if I only write a few words.