When complaining in English, it helps to be polite. This page will help you with this important English speaking skill.
In a shop
You're in a shop and the assistant gives you the wrong change.
"Excuse me, I think you've given me the wrong change", or "Sorry, I think this change is wrong. I gave you $20, not $10."
In a hotel
"Excuse me, but there's a problem with the heating in my room."
"Sorry to bother you, but I think there's something wrong with the air-conditioning."
"I'm afraid I have to make a complaint. Some money has gone missing from my hotel room."
"I'm afraid there's a slight problem with my room - the bed hasn't been made."
When people apologise, they normally say "sorry" and offer to put the situation right.
"Excuse me, but there's a problem with the heating."
"I'm sorry - I'll get someone to check it for you."
or "Sorry to hear that - I'll send someone up."
Although you may find it strange to use the word sorry when you complain, English speakers consider it polite. It will help you get what you want!