If your applying to go in as an Able Rate learn to recognise all the ships and know the names (destroyer, frigate, scimitar class) etc, learn who our first sea lord is ( due to change in April) learn the dates the RFA, Navy and Marines were formed, roughly know the rank structure and read/remember the data sheet about your specialisation and training length tbh that should easily be sufficient... when I first applied as a rating I'm surprised how I got away with knowing next to nothing.. though don't hold me accountable for the above advice it's up to you how much you learn and the more you learn about the Navy before your training the easier life will be.
The History of the Royal Navy can be traced back to before the ninth century AD. However, the present Royal Navy was formally created as the national naval force of the Kingdom of England in 1660, following the Restoration of King Charles II to the throne. In 1707 it became the naval force of the Kingdom of Great Britain after the Union between England and Scotland which merged the English Navy with the much smaller Royal Scots Navy, although the two had begun operating together from the time of the Union of the Crowns in 1603.
Although no defined moment of formation, it's still something the OP could have learned about.