Applying for your first job is always scary for anyone (shy or not). I always try to trick myself into thinking I'm not nervous and go in with the confident attitude that I already have to job. (confident, not cocky, there's a difference.) I find that the more nervous I am, the more likely I am to stumble over my words and mess up.
Preparedness is probably the best advice I can think of for any interview or opportunity. Know the company you're applying for, understand what they do and think why you want to work there and why you would be a valuable assett to them. Even if you have no job experience, pull from non job related experience. Example: "I'm very punctual at all of my appointments, I like to be prepared (if you're in school you can refer to school work and projects here, "I pay attention to detail".... Etc... And give examples. But think about things that apply to you and your experience. Anyone can say any of the above but of its not true, the employer will know it, so think of ones that do apply.
But the more you're prepared, the less nervous you'll be.

As far as if you don't get it.... I can't tell you to not let it bother you because it probably will, it's a normal reaction. No one likes to be rejected. But just take it as a learning experience. I've always been told that you should try for any job/volunteer opportunity you're interested. Even if you don't get it, it's still interview experience. Since this is your first time doing this, you could chalk it up to practice and move on to another opportunity you're interested in. You'll probably to better next time because you'll know what to expect.

Good luck!