Brainstorm to find the main idea or theme of your memoir. List things you remember, things that are important for one reason or another. Talk to friends or family who can help you fill in any "blanks" about the time you want to write about.
Write your first draft. Some writers start at the beginning and move to the end, others write in sections and then piece it all together later. Do what works for you. The most important thing is not how you write but that you simply write.
Indulge your work by free-writing. Make a list of words or short phrases that spark memories for you. Set a timer for 10 minutes and just write whatever comes. Forget about spelling, punctuation and even grammar. Just write. Afterwards, look over your words and see if there are any ideas or phrases you can develop later.
Revise your work. Once you've got it written down, rework things. You might want to take a few days between writing and revising so that you approach it with a clear head.
Read your work aloud to yourself. Listen for passages that flow nicely and mark passages where you stumble. If the words are difficult to speak, they will probably be difficult for your reader.
Share your work and get feedback. Seek guidance from a teacher or a writer, or join a writing group. Be brave.
Polish and make additional revisions. Put your pages away for a few weeks or months, and then have another look. You will likely see things you want to change. Once you've made the final revisions, you can explore publishing.