There are a few things that are best to avoid when journaling. It's not that they are necessarily wrong per se, but, they could impact your journaling practice and the effectiveness of it.
One important thing to avoid is to not judge yourself and what you are writing. Journaling is a safe space for you to write down what ever you feel you need to write down. This could be your thoughts, emotions, feelings, things that have happened that day, things you want to have happened, or anything else. It's a place for you to vent, to clarify and understand yourself, and to go on a journey of self discovery.
Another important thing is to keep it simple. Don't try to have perfect grammar, it's not the point of journaling. Don't try to copy someone else's style or write in certain styles other than your own, just be yourself. And don't try to fill out a whole page or multiple ones every time you journal, just write as much as you feel you need to, even if it's just one sentence.
Here is a longer article I wrote on this site's blog that covers more things to avoid and things that can help your journaling practice: The Essential Guide To Journaling: Dos, Don'ts, And Tips For Success