Just as a starter, you should know where your training will take place, how long it will take and what it will include. Give examples of the ships on which you are likely to serve - what type are they, name a couple. What will the main duties of your job be? How much will your salary be?
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A lot of the interview will actually be about you, your education, jobs, family, hobbies etc.
Do you work well in a team? Are you fit, trustworthy, motivated?
What motivated you to apply to join the Royal Navy? What appealed to you about the ME apprenticeship?
There you go.