WHEN YOU ARE WRITING IN ENGLISH YOU NEED TO USE "AM" AND "PM"
In Spain it is very common to use the 24 hr clock but in the UK this is less common. So when you are writing an email in English you should use the 12 hour clock.
So, for example: "I will see you tomorrow at 9am" or "We will go for a drink in the pub (no se dice "puff") at 5pm". People will understand what you mean if you put "17:00" in your emails so don't worry if you prefer the 24 hour clock.
Hope that helps!
Soy el director de Daniel Smith + Partners. Tengo más de 10 años de experiencia enseñando inglés a españoles.