You can either free-write (write what comes to mind), set a specific topic for your journaling, or use guided journaling guides! There are many of these you can find on the internet. You can also buy guided journals from places like Amazon, Target, or Walmart.
There is no set structure you must follow in order to journal. Journaling is all about being honest and open with yourself about your thoughts, emotions, and experiences. Find a comfortable place and allow your thoughts to speak freely.