The car of Mary broke down, so she arranged somebody to repair it. Yesterday a mechanic came and took the car to the garage to repair it.
Mary had the car fixed yesterday.
It means: Mary arranged for somebody else to fix the car. She did not fixed it herself.
Have something done is used to say that you arrange for somebody to do something for you.
Take a look at it:
Mary fixed the car.(she did it herself)
Mary had the car fixed.(she arranged for somebody to do it)
Be careful with the structure, the word order that should always be:
have + object + past participle
see another example and pay attention to the structure to be used:
I have my car painted very soon.
Alternatively, in informal spoken English you also can use "get something done" instead of "have something done", so you'll have:
When am I going to get my car painted?
Hope it helps you. See you next time.