It's important that you're yourself when you meet people. If you try to act differently, after a while they'll know. You will find people that you'll get along with and who will appreciate you for you. You can try to start a conversation with people. Not everyone would like people who act 'cool'. You're good just the way you are and it's okay if it doesn't go so easy for you. You're not the only one. You'll meet people who think the same thing as you.
It may sound cliche, but just be yourself.
Don't overact to try to make yourself seem like something you are not.
However, if you simply want to appear confident, make direct eye contact with people, stand up straight, practice smiling a little bit more, and shake people's hand firmly. Those all give the impression that you're a reliable and charismatic person.