I get really nervous when I talk to people?

Im a 17 year old guy and I don't know why, but I get extremely nervous when talking to people. I can be around a ton of people and feel okay, but something as simple as ordering food makes me really nervous, and I think the cashier can usually tell and it makes them uncomfortable. Like today I bought a backpack for school (Starting my senior year tomorrow woot woot!), and I had to ask the cashier to get it for me from behind the counter and I was so nervous. I don't worry about people judging me and stuff, because honestly I don't know what there is to judge. I mean I take care of myself, I'm healthy weight, I'm 6'2, I wear nice clothes, and I think I'm decent looking (not being arrogant, just making a point). And on top of that, I'm really good at admitting my flaws. I love meeting people, and talking to people when I feel good, but the nervousness just gets the best of me! Somebody help please! I want to be able to enjoy life!