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?
in order to pass themselves off as ordinary citizens. In most countries police officers are given exemptions from certain laws to perform their duties. For example an officer may use force if necessary to arrest or detain a person when it would ordinarily be assault. Officers can also break road rules to perform their duties.
Etymology.
The word "police" comes from the Greek "politeia" meaning government, which came to mean its civil administration. Police officers are those empowered by government to enforce the laws it creates.
Police officer
A police officer, also known as an officer, policeman, policewoman, cop/copper, garda, police agent, or a police employee is a warranted law employee of a police force. In most countries, "police officer" is a generic term not specifying a particular rank. In some, the use of the rank "officer" is legally reserved for military personnel.
Police officers are generally charged with the apprehension of criminals and the prevention and detection of crime, protection and assistance of the