Journal writing assignments are both the hardest kind of writing and the easiest: Easiest because almost always when instructors ask you to write something in journal form, they are not looking for "polished" writing--error-free, finished, or completely developed. They are looking for rambling, "messy" thinking--your process--as you study what it is they've asked you to study. Often the journal assignments are very open-ended. That's why they're sometimes the hardest kind of writing: sometimes you don't get much direction.
If you're having trouble getting started, just start writing, fast. If your inner voice is saying "I can't do this. I can't. I can't." write "I can't do this. I can't. I can't." And keep recording what that voice is saying, write, write, write. Don't worry about punctuation, spelling, whether or not what you're saying makes sense. Don't stop. Don't go back and look at what you just wrote. (That voice--the editorial voice--is forbidden in free writing.) Keep writing and keep refocusing on the subject assigned. More and more ideas will come out onto the page.
Usually instructors are really looking for two things: