These are some pointers taken from this site:
Take classes to increase your self-confidence.
Get help through a psychologist or other medical
Stop seeing people as threats and stop seeing yourself as a victim; if you don't want to do
something, just say so.
Own things and do them well;
stop procrastinating and stop "forgetting" things on purpose; people do notice and it wears thin.
Get positive. Being negative all the time is not only a real drag on those around you but it is stifling you and your abilities.
Realize that you are responsible for you and that only you' can make your own life a better, more pleasant one.
Stop criticizing other people, especially those in authority. It doesn't make them smaller but it does make you smaller; it also makes you boring to be around and others are very wary that they're going to be the next target of your nasty jibes.