Know what to expect in the first 2-3 years after Officer Training and get hold of a copy of Ships Guns and Missiles. Know the difference between, say, a Type-22 and a Type 23 Frigate.
Typically, they will ask you one question then a follow-up on the same subject to test your depth of knowledge. For example, "What types of aircraft do we use.. " followed by "What engine does a Sea-Harrier have"...etc.
Be yourself and good luck