A journal can be the start of a book or something great. Or just help mark milestones in your life.

How to Keep a Journal
Any notebook will do. Doesn't have to be fancy. Write daily. 
Writing by hand works better.  /we are more connected to our thoughts when we write by hand. Studies have shown more areas light up in our brain. Write for about 15-20 minutes every day. A set time helps form habits of course. And it will take that long to get deeply into your thoughts and articulate them perhaps. You can write your profound thoughts down that no one wants to listen to! Or your solution to someone's problems, including your own. Ideas: What scares you? What makes you laugh or cry? What is happening in your life that makes things tough for you or challenge you. You can time yourself or write during a time that allows you unended space of time.   Try freewriting, and write without stopping to think or edit. Don't worry if it sounds like nonsense or you make grammatical mistakes. Don't set limits on yourself as to what you write about. You can always destroy it later if you don't want someone else to read it.  There really aren't rules. Maybe determine your purpose for keeping one first. Is it to vent your feelings, leave profound thoughts behind, have a breeding ground for writing a novel? 
Whatever you decide, writing a journal is therapeutic, mind-expanding, and worth your while.