Showing confidence in your interview doesn't always mean you are confident on the inside. I have yet to meet somebody who can tell me they were 100% confident in every interview they ever had. In fact, I have yet to meet somebody who can tell me they were 100% confident for ANY interview they ever had! 

Everybody gets nervous. Everybody. So don't let it bother you if you're getting the jitters, as you're not alone. Usually an interviewee's nerves are present at the beginning of the interview, and you can iron them out as the interview progresses; but this is only if your preparation is on point.

There are some people who appear as if nothing fazes them, but they are just disguising their nerves and acting in a confident manner by keeping in mind a few important points:

1. Preparation is key - This is the first and most important thing to remember about your interview. As you answer your interviewer's questions, you'll start to realize that you have prepared well and the answers will just roll off your tongue. Confidence will then start to grow and any nerves will begin to go out the window.
2. Practice your handshake - When you meet somebody for the first time (in this case, the interviewer) and you offer a half-assed handshake, it doesn't come across in a mature or professional manner. An interviewer might think that you lack interview experience or confidence in yourself. It can also imply a lack of respect. Practice your handshake with friends or family. Find the balance between strong and weak, but a good firm handshake is always the way to go.
3. Keep eye contact - This will help you increase your confidence throughout the interview. Practice in your everyday life. Keep good eye contact during conversations you have with friends or family or anyone else you encounter. Remember, good eye contact while listening is just as important as when speaking. There is nothing that comes across more confident in an interview than good eye contact.
4. Listen attentively - Ensure that you're listening to everything your interviewer is saying. That way you can really answer the questions being put to you instead of sounding almost robotic with rehearsed answers. Your preparation is your study. What it should do is remind you of things to say, not to recite things word for word. You want it to come across as natural. If you listen attentively to your interviewer, you could hear a joke and laugh, or something they say could spark a conversation relevant to the interviewer; which will in turn create a comfortable situation between both of you and improve your chances. Listen to absolutely everything that is said to you, take it in, then offer your response. If you are contemplating your next answer while they are speaking, then you aren't listening. Don't make this mistake!
5. Talk slowly - A confident person is not afraid to say something like "that's a great question, let me think about that for a moment", while giving themselves time to think and then relaying their answer in a nice, easy way. Speaking quickly can come across scripted, and valuable things could be forgotten because the interviewee might be rushing and not thinking properly.
6. Body language - If you are slouching in your chair throughout the interview, it will look like you are uninterested. Sit in an 'open position' or a 'power stance'. An open position is where you look welcoming and respectful of another party in a conversation. Doing this invites your interviewer to the conversation and sends the message that you want to be there. A power stance takes up space in your chair, making you look big and confident instead of small and fragile. Talking with your hands conveys enthusiasm, which will unlock more of your confidence in your interview. More importantly, you won't fidget as your hands will be tied up with talking.
7. Smile - Smiling puts everybody at ease, and there is nothing better to build interview confidence than to put you and your interviewer at ease together with some human politeness. A smile goes a long way and confident people smile.