No need for a pro for this..
Stick with a good brand, or at least a well reviewed product and you'll be fine. Wireless sensors make these things really easy to install.
I personally prefer hardwiring, but it isn't always possible or feasible. I
Something like this GE
http://www.smarthome.com/ge-security-80-860-4-concord-4-wireless-crystal-alarm-kit.html or the Simon mentioned above are great starting points.
You can add sirens, lights whatever you need.
I did a basic system for my Aunt with something similar; she wanted to know that the garage was secure but not pay for any monitoring.
Installed a basic GE panel with several door/window sensors as we as a motion detector.
I added a couple 120db sirens (they are very painful in an insulated 2 car garage...