I have an exceptionally confident (but not in the slightest bit arrogant) 16yo, after a lifetime of struggling with my own confidence. Honesty, I am in awe of her confidence. She is willing and able to talk to anyone and she will give anything a go. She is confident in her own abilities but not afraid of failing and can laugh at herself easily. She is self aware enough to know her own strengths and weaknesses, and be comfortable with those, and she is able to celebrate and tolerate the strengths and weaknesses of others in equal measure. And she cares about living up to her own high standards of ethical behaviour, rather than about what others might think. I wish I had some good tips to pass on to you, but tbh, I think it's largely a question of luck! I strongly suspect that there is an innate element to confidence that kids either have or they don't.
What I can say is that dd has always had lots of time and attention from us as parents, and also from the other adults in her life. We have consistently talked to her as an equal, taken an interest in her interests and respected her views and opinions.
She has had lots of unconditional love, expressed both verbally and physically, and we have tried very hard to respond to her emotional needs. I am far from a perfect parent and get things wrong all the time, but I have always tried to acknowledge when I've got things wrong and apologise so that she understands that nobody is perfect and so that she knows that she isn't necessarily responsible for my bad moods etc.
This might be controversial but I actively sought out opportunities for dd to struggle and fail when she was younger so that she learned not to fear failure in the way that I do. She is naturally very good at most things she tries, often without much effort, so it was really important to me that she learned how to strive and persevere with the things that didn't come so easily to her. She learned that hard work can enable you to master things that you might initially find difficult, and that has been hugely important. Extracurricular activities were great for this, especially dance, drama and swimming because dd was not a natural fish!
Over the years, we also created lots of opportunities for her to talk to people from different ages and backgrounds. We would chat to people on buses and in queues etc, make new friends on holiday etc. This comes much more naturally to my DH than it does to me, but I tried really hard to emulate him and dd has always been very open and friendly. (Obviously, you have to balance this with teaching your child how to be safe around strangers as well!)
Finally, I used to regularly take opportunities to push dd out of her comfort zone by getting her to try new things. When she was really tiny, I encouraged her to ask for things in shops, pay for stuff, try new activities etc. We arranged for her to spend a day in an overseas school when she was 7. All of these experiences helped her to develop her self belief.
I do believe that all of these things helped, but to a great extent, I also believe that dd was just born confident and that enabled her to make the very most of these opportunities.
Good luck with supporting your dc. I'm sure she will be fine!