The first step toward building self-esteem is getting to know yourself and what you stand for. In other words, self reflection. There are lots of ways to do this. Try some of these suggestions:

Start with something simple, like keeping a journal. Journals help you express your feelings! When you write out your emotions, you will understand them more clearly.

Sit and think for a few minutes each day. This can help you understand yourself better. You may want to think at night, when everything is peaceful and quiet, or in the morning, before you start your day.

Yoga is a great way to get in touch with yourself. In yoga, you exercise and practice breathing in a way that clears your mind and restores your balance.

Do what you love to do, whether it's painting, playing an instrument, or riding your bike. When you find something you love, follow that idea and don't stop until you've reached your goal. And you don't even have to stop there! There's no limit.

Focus on the inside and try forgetting about the outside for awhile. That doesn't mean you shouldn't comb your hair or brush your teeth, but you could go a day without the makeup or something else that's not entirely necessary.

Spend the time you saved thinking about how you could improve yourself on the inside.

Try being kinder to people. Say something nice to that lonely girl in the corner. You will discover the compassion in your heart.

- Source: Girls Know Best 3: Tips On Life & Fun Stuff To Do

A book that you might find helpful is :

Stillman, Sarah. Soul Searching: A Girl's Guide to Finding Herself. Oregon: Beyond Words Publishing, Inc., 2000.