Yes, it's already time for high school juniors to start planning their summers and their college essays.
You can't write a good essay unless you have something to write about. And you can't figure out who you are without journaling about important experiences.
Trust me. I've helped students do it. The first phase is experience. The second is journaling. The third is your first draft.
Here are some great tips to get started: https://bigfuture.collegeboard.org/get-in/essays/8-tips-for-crafting-your-best-college-essay
But the best thing of all? Get together with a specialist who will help you plan an authentic experience, coach you through your journaling on it, and take you through the entire dreadful experience until you can hit "SUBMIT" and feel just like you do after a great workout. DONE!