My little brother wants to be a police officer when he gets older.
That is great! A police officer is also known as an officer, police person, policeman, policewomen, cop, police agent or police employee. 
Wow so many different names. Is it hard to become one?
No it is not too difficult to become, one but it does require a lot of training. Some police may be trained in special duties such as counter-terrorism, surveillance, child protection, VIP protection, civil law enforcement and investigation techniques into major crimes including fraud, murder, and drug trafficking 
It seems like a dangerous profession to be in.
Yes it can be but they have sworn an oath to protect and serve the citizen in the country. We are lucky to have good law enforcement available to us to keep us safe and the community in order. :)
You are right about that. Do most countries have police officers?
Yes, although in most countries the term "police officer" is generic because it does not specify their rank. 
I see. What else should I know about police officers that I can tell my brother?
Not all police officers have a uniform, some dress like normal people on the street in order to dissimulate as ordinary citizens