Here are some key ideas to get a confidence boost:
Take action
Most of all the confidence you'll ever build comes from taking action and at least small risks. You can't build your confidence by sitting back, feeding your doubts and hesitating endlessly.
Learn by experience
Accept mistakes as part of the learning curve. Use rejection as a way of boosting your confidence. Move forward always.
Gather your tools
Confidence building comes from gathering all you need to be successful. It can be new skills, key information, tools, sources of support, etc. Find out where they are and get them.
Focus on power
Power and confidence are one and same thing. Get familiar with that word and find out what your sources of power are. Use them and build yourself a personal foundation which can't be shaken.