Merriam-Webster’s Dictionary’s definition of confidence is “a feeling or belief that you can do something well or succeed at something” or “the feeling of being certain that something will happen or that something is true.”
The issue here is that neither definition tells HOW to be confident.
It is definitely hard to feel confident when we are not prepared. By doing research on a project or speech and dressing in an appropriate manner, our confidence level should increase.
So why do we still express self doubt? How do some people come off as so sure of themselves and others feel very insecure and nervous?
We really need to look deep into ourselves and figure out why we feel this way. It may be necessary to consider the way you were raised or what you learned in school or on the job.
It could also be that someone commented that you could have done something better or in a different way. If it is the latter, it is necessary to analyze why the person said this. It could be they were trying to be helpful, but it also could be they were jealous or coming from their own insecurities or lack of feelings of self worth.
Write down all the things that come to mind and recognize them for what they are. Also remind yourself of areas where you did feel insecure but eventually overcame those feelings.
Trying something new can help our confidence level but it does come at a risk.
It is the opportunity to succeed at something we never tried before, but it could make us feel even worse if we fail. If you try out small risks (or I like to say daring myself) first and accomplish them successfully, it will definitely help.
An example could be that you get very nervous public speaking.
Invite a few friends over to hear a short version of your speech. You are still in your comfort zone because you trust these individuals, but you are also taking a risk that they may not react the way you want them to. It is a gamble, but it is a small one, and most likely you will do a great job.
Seek out others whom you perceive as having mastered being confident.
Ask them what challenges they overcame and if these can apply to you. Being humble and asking for guidance can really help you shed light on strategies that might work well for you. The advice they give may not help with a current situation, but I am sure that someday you will put it to good use.
If nothing else, think of the phrase “fake it till you make it.” 
By showing a more positive attitude, you can actually become more confident. Taking deep breaths and smiling will help you in this endeavor and over time your confidence will build and mature.