The easiest thing is to talk it over with the teacher, but not when you're upset - ask her to go through your work together and show her essays from classmates who got higher grades. If she explains the criteria and it becomes clear the grade really is unfair, then you can go to your class supervisor or the assistant principal - they'll help sort out the situation or suggest redoing the assignment. The main thing is to approach it not as a complaint, but as a request to understand what happened, that way you're more likely to be heard.