If you 'have/get something done', you get somebody else to do something for you.
The grammar for this is pretty simple: Have (Get) + object + past participle
NOTE: The differences between have and get something done are that have is slightly more formal than get, and that get is more frequent than have in the imperative form.
- I'm going to have my hair cut.
- I got the bill sent direct to the company.
'I had my watch fixed' tells us about the past.
'I am going to have / get my watch fixed' tells us about the future.
Did you have your car fixed?
Using have something done for bad things:
Jenny had her car stolen.
Asking or instructing:
I’ll have Harry book you a taxi.
Talking about an experience:
Her story had us laughing so much.