Have: [have + person + verb] - means "to give someone the responsibility to do something."
Examples:
1.Please have your secretary fax me the information.
2.I had the mechanic check the brakes.

Get : [get + person + to + verb] - This construction usually means "to convince to do something" or "to trick someone into doing something."
Examples:
1. Susie got her son to take the medicine even though it tasted terrible.
2. How can parents get their children to read more?

Sometimes "get someone to do something" is interchangeable with "have someone do something," but these expressions do not mean exactly the same thing.
Examples:
1. I got the mechanic to check my brakes.
2. I asked the mechanic to check the brakes.