Look for a diary at a book store. Find a diary with a design that fits your personality. Make sure you find one that comes with a lock, because it's better to keep your diary private.
Find a careful place in your room to store your diary, as well as your diary key. Make sure you keep your diary in a place that's easily accessible to you, but not to others.
Encourage yourself to try to write in your diary daily, or at least a couple of times a week. Try not to get out of the habit, get in the routine so that writing in your diary feels natural to you. You'll be grateful you did this later on when you want to remember your emotions and feelings of a time before. It's YOUR personal diary.... it's all about you!
Talk about yourself and your feelings. You can never say enough about yourself.