Low self esteem usually originates from early life experiences that lead a person to view themselves in a negative manner. For example, if a person is exposed to situations that offer negative feedback about self worth (e.g. rejection from friends, certain negative parenting styles) then beliefs about the self begin to form that reflect these experiences.
Obviously this is a very brief and general answer and to better understand how low self esteem may have developed in your own situation I recommend you take a look at this excellent Cognitive Behavioral Therapy (CBT) based self-help program that is designed specifically for problematic low self esteem
It is completely free and will provide you with detailed information on how low self esteem develops, how it is maintained and what you can begin to do about it.
I hope this has been of some help. Please let me know if you have further questions or would like me to clarify any part of my answer.