Simply stated, keeping a journal helps you to remember.  Remembering helps you to reflect on past experiences, and gain insight into yourself and your problem solving techniques.  Journals also help you to keep track of every kind of thought process and experience which has taken place over time, in your life.  Why is this helpful for you?  It is hard to be introspective.  It’s difficult to take that long look at ourselves to see who we are and how we act in various situations.  For example, if you want to record only your first experience with anything, a rock concert, a flight over the ocean, a meeting with someone you idolize, the pressure you felt when you took your drivers test:  later, looking back at what you wrote, and how you felt, and perhaps, writing again from your more recent perspective, gives you a vast knowledge of yourself and your personal growth.  Some people use journals to record outside activities, perhaps the first signs of global warming in your environment, the first problem with getting enough water to drink in your state, or the first presidential campaign in which you took an interest and voted.  It might be a good idea to write down what the candidates promise before the election, what actually happens while they are in office, and what their actions did to shape the world, that lasted long after their term had expired.  Upon reflection, you have facts that will be invaluable to you.  I have just touched on a few of the ways to use a journal.  The possibilities are endless.  Pick one up almost anywhere that notebooks are sold.  Leather Bound varieties, and more elaborate styles are available online at amazon.com and so many other sites, and always on auction sites.  I got most of mine at Barnes & Noble booksellers.