Building self-esteem is as simple as changing the way you talk to yourself.
Its also as hard as the way you talk to yourself.
A friend once said, "Positive thinking is powerful, but negative thinking is 1000X more powerful"
Its a matter of retraining your brain as to how you think. A couple of methods:
Mantras - create a mantra (or two) for yourself and tell yourself that whenever you look in the mirror. It works better if you say it out loud. That way your brain processes it verbally as well as in thought form.
"What's good about me" lists. Create a journal about what's good about you. Add to it. Read it frequently.
Find something you're good at and get better. Find something you'd like to do and get good. Find something that you want to do and excel at it. Learn a new skill.
Ask your friends, "what is it about me?" and write it down.
Wash, rinse, repeat. The messages we hear when we are very young permeate into our daily lives as adults. We need to rewrite those messages.