Job interviews begin the moment that you walk in the door and end when you send a thank you email afterwards. At every moment, it's important to be putting your best self forward in order to show that you are a great candidate for the job and that you are interested in the company. Perparation can help lower your stress level. Practicing questions outloud beforehand can help you focus on what's important in what you want to say. It can also be helpful to write your questions down beforehand so that you are ready with smart and relevant inquiries.
Technical interviews can be intimidating but are really to show how you think and collaborate. If you can feel confident talking through your thought process and expressing the way you're approaching the problem, you will be ok.
You're not only selling yourself, though. It is important that you ask questions of the interviewer as well. This is your best oppurtunity to gauge whether you think that the company would be a good fit for you and your career goals.