If you are joining as an officer I suggest you wear the brightest coloured moleskin trousers you can find or failing that cords. They should preferably be orange or pink but any bright colour will do, mine are lilac.
Top it off with brown brogues (preferably suede), a checked shirt (don't even think about one that has buttons on the cuffs or has a pocket), your school tie or another suitable one from your polo club or rowing squadron, finally you will need a tweed jacket, it should be two sizes too small and borrowed from a deceased family member.
A panama hat is recommended for summer but since it is autumn a tweed cap is acceptable. Obviously I don't need to explain the importance of selecting the correct umbrela.