1. As others have said, practice is the only real way to get better. If you can carve out the time, interviewing even if you aren't actively looking to change jobs is a good idea.

2. Specifically to your question about proper narrative, people often recommend the STAR (Situation -> Tasks required of you -> Actions that you took -> Results that you achieved) framework for answers. It can definitely sound a little robotic if over applied, but I think it's relatively useful especially for new people. It's also helpful to start by taking a moment to gather your thoughts ("I would love to take a brief moment to think of an example"), and work backwards from the Results you want the interviewer to remember.