First begin each day by looking yourself in the mirror and saying out loud," I am a valuable person. I am on this earth for a reason weather I know the reason or not. Today is one more step on my journey of discovery. I don't have to be perfect and I can be happy today." It may seem awkward and silly at first, but over time it will become the new tape in your head.
The second step is related to how we treat others. It is said if your want a smile, give someone else in need one. It is a truth that you get what you're willing to give. Giving to others smiles, an encouraging word, a hand up creates a spiral of positive events which often grows. Even if the person doesn't reciprocate, you've fed your own soul and in doing so improved your self-esteem. It's can be helpful to remember that as bad as you may feel about yourself there are other people who struggle as you do and could use your help. Giving in whatever way, helps you get out of yourself enough to understand you're not really alone.
The third step is to set reasonable attainable goals and go accomplish them. Think small here. Saving the world is too broad a goal. A good goal can be as simple as learn something new each day. It can be something as trivial as a new word. The point is to make a habit of setting goals for you every day and attaining them. It's not that long-term goals aren't important but rather that small successes are the path to all long term goals. You don't just wake up one day with a degree. Thousands of small daily goals get you to that coveted long term goal.
These are just three small steps of many ways to improve your self-esteem. As simple as they seem, practiced daily they will make a big difference.