Use HAVE/GET + object + past participle to say you arrange for someone else to do something for you. You don't do it yourself - you pay/ask someone else to do it.
Quick tip: HAVE/GET + object + past participle = someone else does it for you. I had my car fixed = mechanic fixed it (not me).
Form: HAVE/GET + object + past participle
HAVE/GET + thing/person + past participle. You arrange for someone else to do the action. The object receives the action.
I had my hair cut. (someone cut my hair for me)
She got her car repaired. (someone repaired her car)
We're having the house painted. (someone is painting it)
He got his teeth checked. (dentist checked them)
Use: Paying/arranging for services
Use when you pay professionals or arrange for others to do work for you: haircuts, repairs, cleaning, medical services, etc.
I'm having my car serviced. (mechanic does it)
She got her nails done. (nail technician does it)
We had the roof fixed. (roofer did it)
He's getting his suit cleaned. (dry cleaner does it)
HAVE vs GET
HAVE is more formal. GET is more informal/casual. Both mean the same. HAVE is more common in British English, GET in American English.
I had my hair cut. (formal/British)
I got my hair cut. (informal/American)
Both are correct!
Common mistakes
Not "I had cut my hair." — say "I had my hair cut.". Causative needs object (my hair) between HAVE and past participle. I had my hair cut (not I had cut)..
Not "I got repair my car." — say "I got my car repaired.". Causative: GET + object + past participle. Got my car repaired (not got repair)..
Practise it
Someone else does it? Use HAVE/GET + object + past participle. I had my car fixed = mechanic fixed it (not me)!