You can start with this sentence: "I have a lot of dreams, but the most important thing to do is to turn them into reality"
Then you can write about some of your dreams: "For example, I want to be an astronaut, a politician, a fire fighter etc."
Then write about the most important dream:" But I have always wanted to be a teacher, because I can help a lot of people through education."
You can cite other examples or inspiration, such as :"One day on italki, I met this rabbit with a carrot who once taught me that everything is possible. If you can dream it you can do it. "
Later on, if you are feeling confident, you can add a twist to it:" With a magic carrot, he prodded me and voila, I suddenly wanted to be an English teacher."
Then finish off with a conclusion, say:" That is why I wanted to be an English teacher." Or something like that.
But to tell you the truth, a 400 character essay, usually takes a good half a day to prepare. Make sure you get at least 6 to 8 hours preparation time. Otherwise your dream will be just that- a dream.