First job search
This is something you'll probably only be able to overcome with time and repetition, but you'll definitely need to work on it deliberately. But you're already aware of your nervousness, which is a major step of it!
As far as repetition, this means that you'll need to be ok with still being nervous for a few more interviews, but also that you should try and get incrementally better with each interview. You don't need to be perfect in the next interview you do. You just need to keep doing better than the last one you did. With that in mind, try to schedule a ton of interviews too, so you can really get a good amount of reps in.
As far as how to reduce the nervousness, remember that you're just gonna have a conversation with a person. Try not to overcomplicate it. Yes, they'll be asking you some difficult questions, but you already know that that's coming. Try to talk to them just like you'd talk to a friend or family member. Seriously – no different. They'd actually prefer that to super-professional talk in a most cases because a lot of what an interviewer is looking for is if they would want to work with you for 40+ hours a week.
So really, try to relax, and practice it just like anything else you'd practice. With some deliberate effort and repetition!