Over coming low self esteem requires small steps. You have to build it up slowly, it cannot happens over-night.
For a start, have a proper posture. When you sit up straight, it makes you feel more confident. Also, tackle simple situations without too much hesitation, it makes you look assertive. Do not speak too soon, a wise person always speak last because he listens to both side of an argument before making his or her opinion known.
People around you will notice your change and comment you on your confidence. You will feel more encouraged and be more confident. This will accumulate over time. But do note, there will be situations that will set you back and there will always be people who criticize others. As long as you knows there will be set backs, you can be ready to handle it.
You can also try hypnotherapy or self-hypnosis, it will take several sessions. Results are relatively good for most but not all.