Do power poses: Mind believes, the way your body acts!! Believe it or not, doing 2 minutes of power pose can increase your confidence exponantially.
Get a hobby: Try something you are passionate about. If you work hard be toward your passion, it will makes you happy and makes you feel confident.
Think of things you are good at: Everyone has their talents and strength, find out about yours and build on those things.
Set some goals: Set small goals such as walking 10k steps, not eating junk food for a whole day, etc. These small goals can make a huge impact on self confidence.
Look at what you have achieved: Make a list of things you have achieved in life. And in time of nervousness, read it out to gain confidence.