Assuming you want tips on writing a memoir as a 5th grader:
First, choose a time in your life that was memorable or had a big impact on you, and start writing about it. It doesn't have to be something big, like moving to a new town or going on a trip - it could be something small, like making a new friend or learning to ride a bike. Just choose a moment that you want to share.
Next, start thinking about the details of what happened. What did you see, smell, hear, and feel? What were you thinking and feeling at the time? The more specific you can be, the better.
Once you have your story and details down, start writing! Begin with a strong opening sentence that will grab your reader's attention, and then keep going. Remember to stay true to your voice, and don't worry if your story isn't perfect - just keep writing and have fun with it.