Any essay or writing that is limited to 250 is tough, since you have limited space to create a well-written piece. I think I understand that you are to create a journal entry about conversation with yourself. I don't know what context this is in...perhaps you have just read a story where the character talks to hmself often? Or maybe this is just an exercise when you are talking or thinking to yourself about an issue or problem.
In any situation, you will need to plan your writing. Think about the topic. Do you talk to yourself? If not consciously, how do you work out issues and problems in your life? Do you practice speeches and or presentations in your bathroom mirror? Do you keep a journal or do you think about things in depth? Maybe you draw or listen to certain types of music when you're feeling pressured, stressed, happy, or angry. Whatever you do to work things out, it is a form of "conversation with myself".
Make a list of things you do that qualify as conversation with yourself, and then list why you do them. You may not know right away without thinking about the "why", and that's OK. Be honest in your journal entry. Then, count your words. If you're over 250, you'll have to begin editing and taking out the unnecessary or repetitive stuff. Chose your words carefully, and have fun! Good Luck!