When we say that we have something done in English, it means that a service is done for us by someone else.

He has just had his car washed.
They had their house painted.
She’s having two teeth pulled out tomorrow.
I had a new heater put in last week.

It can also express an unpleasant situation:

I had my car stolen last week so I need to travel by bus now.
Kelly had her purse stolen while she was shopping.

When we use the expression get something done, it is often similar to having something done but it has a feeling that something must be done, there is an obligation.

I need to get these exercises done by noon or else I’m in trouble with my teacher.

The verb get can also be used instead of be in the passive, mainly in spoken English.

She got hit by a car when she was crossing the street.
The thief got arrested when he was breaking into that house.