Lot of great advice here already, if you use the search function above. It boils down to this:
Dress neatly and respectfully.
Make great eye contact, offer a firm handshake, sit up straight.
Speak slowly, steadily, confidently — without all the teenage fallbacks such as “I was all, like, you know...”
Tell your unique story of why you want to serve on active duty, why this particular service, why this particular accession path. Say what’s true and real to you, not what you think they want to hear.
Practice with someone you trust to give you real feedback. And if you’re only starting your practice interviews now, well...
Above all else, be yourself. Veteran interviewers have a way of seeing through the BS.