What does it mean to "have something done"?  This means that you want someone, i.e. a trained professional, someone specialized in something, etc., to do a service for you that you either can't do yourself or don't want to do yourself. 

A good example of this is having your roof replaced. Even if you know how to do it correctly, maybe you don't have enough time to do it or you have hurt your back and can't do it.  In this case, we can say "I need to get my roof done."  This simply means you will contact someone who can replace your roof for a fee. 

Another good example is if you want something done professionally.  For example, if you are going to a wedding and would like to have your hair and fingernails look really nice, you may not have the ability to make them look this way, so you would "go and get your hair and nails done" at a hairdressers' and then get a manicure