It's not clear from the assignment whether your task is to write a non-fiction or fiction piece. Either way, you can interpret the idea of a journey in many ways. For example, a journey can be a trip to a physical location, or it can also be a metaphorical journey. Examples of a metaphorical journey include finding out something new about yourself, or about a loved one, or seeing a place where you've been before in a new way.
As you prepare for this assignment, perhaps go over old photographs and think about journeys, both physical and metaphorical, that you've made. You can think about trips you've taken, friends you've gotten to know, or ways you've gotten to know yourself better. When writing, think about starting with a hook that grabs the reader, and try to show the reader the changes you went through on your journey by writing dialogue and description rather than by simply telling the reader what you experienced.