What do you feel you need to write down? For me, I always feel that it's not necessarily about what I should write down as much as it is what I should not write down, or not focus on.
There are basic things I like to keep in mind about journaling, such as, it's not a diary. It is not something that I must do every day. It is also not a place for me to write down everything that has happened that day.
Journaling for me is the place I go to understand myself better. To make the things that are foggy and unclear in my mind, clear. For the most part, it works.
Of course, it's not a magical solution to my problems or what ever I am facing or feeling. But it is one of the best outlets. It's a place where I can just write down what ever I am feeling, with no judgment, and no worries about how it may sound.
Another important thing to not focus on is your spelling, grammar, and how what you are writing down sounds. Those things are not important. What is important is what you are writing, not how.
As for what to actually write, it depends on what you are feeling, and what brings out the most from you. For me, I like to ask myself the question, "Where am I now?". This question makes me take a step back and realize if I am stuck in the same place, or if I moved forward or backwards. From there, I ask more follow-up questions to further explore my mind, such as "Why are you still here?", "Why did you go backwards?".
These questions can get my mind, thoughts, and emotions flowing. But that is just me, everyone my have different prompts that work better for them, so it's just important to explore the different prompts, or make up ones yourself, and find what works best for you.
If it is of any interest, here is an article on this site's blog that I wrote about morning journaling. It has a list of 100 prompts to use and to get inspired from for your journaling practice: 100 Morning Journaling Prompts: The Morning Habit That Can Set You Up For A Better Day