I always like to start by asking myself, "Why am I journaling?". If I chose to start journaling and to start writing, there must be a reason for it. If I can understand that reason, even if i just partially understand it, I can use that as a guide to asking myself further questions.
There are many questions you can ask yourself, some people like to stick to the same questions, some like to ask different questions each time, and some like to mix it up. There aren't really rules or "right" ways of journaling, it just depends on why you are journaling and what you hope to get out of it.
Based on the reason you are journaling, you may want to try different journaling types, such as mindfulness journaling, gratitude journaling, or reflection journaling. There are many different types, and each may have different prompts that can help get you started and get your thoughts flowing.
If you are doing morning journaling, or are considering starting to do it, here is an article about how to start a morning journaling habit, which includes 100 prompts.