The first task is for you to fully realize how precious you are. I'm convinced if you (or any of us) knew how others feel about us, it would be mind-blowing! There are, I think, lots of people in your circle who think very highly of you, and you don't even know it.
Second, it would be valuable to involve yourself in projects that you have strong feelings about. Whether it is a faith project, charity or political cause (etc.), you can gain confidence by becoming an expert on what you have serious interest in. It is much easier to be vocal when you are comfortable with a subject, and that allows you to be more confident.
Third, it would probably help if you learned how to handle people who have outgoing and boisterous personalities. Learn about "power phrases" and how people use them. Understanding this will give you the confidence to not be intimidated by the people around you who tend to dominate the conversation. We all have these people in our lives, and understanding how they operate will definitely help. The internet is awash with info on this.
Know as well that what other people think of you, whether bad or good, does NOT have to become YOUR REALITY. Someone elses opinion of you is simply that -their opinion. You cannot control what other people think. You CAN control how you react to them. Never let someone elses opinion of you become so important that you lose sight of the truth! There are so many people out there who disquise their own insecurities by trying to belittle others. See it for what it is (a sad power play) and don't rent space in your head to these people. Ignore.
Also, please know that being shy is not necessarily bad. I know by your question that you seek to overcome this, and that's fine as well, but I don't see shyness as negative. Often the person who speaks LEAST is the most interesting in the room!
Best...
