Whether it's getting up in front of class, becoming the de facto leader of a group project, or just wanting to appear badass, being truly confident is never going to be 100% attainable, but here are some things you can do to get you to the 99%:
1. Don't be afraid to ask questions.
No one is born knowing everything; we just have different aptitudes for understanding concepts the first time. Adults seek the input of others, and know when to ask for help; children assume they know everything, or that asking for help is a weakness. Socrates (Plato's literary character, not real guy) was the perfect, if a little bit annoying, example of someone who was confident in his knowing nothing, always asking for others' opinions: "The only true wisdom is in knowing you know nothing." Asking for help also shows that you have good problem-solving skills. Don't be afraid to say you don't know.
2. It's in the walk.
I have two modes of walking, depending on my mood: either very quickly with a stark purpose, or a knowing, carefree swagger. Confidence can manifest itself in many forms. Amy Cuddy's famous TED talk about how your body language shapes who you are is still extremely relevant. I find myself doing the Wonder Woman pose so often that my friends make fun of me for it, but it doesn't bother me at all; in fact, it brings a smile to my face.
3. Faking it 'til you make it is a real thing.
The first time I was elected to lead a high school club, my first thought was how the hell am I going to pull this off? Turns out that if you can put on an authoritative voice and learn key things as you go along, then suddenly you're the person everyone thinks has it together. Most of my growth has come from putting myself out there and learning from my mistakes.
4. LAUGH at your mistakes.
You're a human being. God knows even the leader of the free world makes a mistake or two, and thousands of people still blindly worship him. It's not a matter of if you'll screw up, it's a matter of how gracefully you respond to obstacles and shortcomings when they arise.
5. Be generous.
Remember that everyone you'll ever meet will have a different version of you in their head -- your mom, your friends, your professors, the neighbors across from you that hate you for no reason -- all have seen you in different circumstances and in different times of your life, and there's nothing you can do to make that image a precise representation of who you really are as a person. Just do your best, be kind, and don't chastise yourself too much if you make a fool of yourself.
6. Be humble.
Even when you think you've made it, it's imperative that you still have a certain amount of humility, and make sure you're approachable and open to new ideas and points of view. My personal brand is one of down-to-earth-ness, always reaching out for help or to help when I'm in a good place emotionally. Every day I reflect on everyone in my life who has helped me get to be where I am now.
The journey from feigning to gaining confidence is going to be scary as hell sometimes, but future you will be so happy you took the first and second and third step. Every time I do something badly, I know with the same amount of certainty that the next time I do it, it'll be a little less bad.
Basically, being confident is slipping on the linoleum and falling on your ass in front of everyone in the room, but you're laughing on the way down.
"You gain strength, courage, and confidence by every experience in which you really stop to look fear in the face. You are able to say to yourself, 'I lived through this horror. I can take the next thing that comes along."