There are many approaches to journaling, but these are ones that resonate with me:
Journaling as mindfulness:
small thoughts and feelings — If it's hard to start by writing full sentences, then just write one word or a couple of words at a time to describe some feeling or thought you're having. Half-formed sentences are great. Eventually, you should be able to start clarifying your thoughts, and be able to write more full sentences.
"wall of consciousness" or "leaves on a stream" — Write literally everything that pops into your head. The goal isn't to have a nice summary or neat resolution, but more to just better notice what's in your mind.
Journaling to process thoughts and emotions:
an "emotional inventory" — List out and try to put a name to what you're feeling, and why. Then think about what to do about it: Is there something that will make you feel better? Should you talk to someone about the concern? Should you alter your behavior?