Choose a diary. You can purchase a fancy diary at most stores, or you can buy a simple composition notebook and decorate it yourself. Another alternative is to write your diary on the computer. This will make it easier to bind your book later if you so choose. You can also join an online diary community.
Make a commitment to write something every day. Diary entries can be as simple as two sentences saying what you did that day, or they can be as long as several pages providing an account of your feelings. If you don't know what to write about, you can always freewrite or make a list of your favorite things, things to do or things you are thankful for.
Keep a list of journal topics pasted to the front of your diary. This list is helpful when you are staring at the blank page and don't know what to write about. Create a list of days--Monday through Sunday--and then give each day a topic. Each day, choose a topic from the list, and journal for 10 minutes on that topic.
Document important events. Use your diary to keep a travel log while on vacation, or use it to record tax-related activities for your business. Write about awards you have won and accomplishments you have achieved. Keep track of your goals and dreams.
Keep a diary that chronicles your child's development. Begin it from the point you find you are expecting, and continue adding to it as your child grows. Your child will treasure this heirloom.
Use your diary as a healing journal. If you are angry at someone, use your diary to write him an unsent letter. If you are hurt, use your diary to write about the experience and sort out your emotions.
Record your family history. Use your diary to talk about your ancestors, your family and your life. Set aside one day per week to devote to writing your memoirs in your diary.