how to use lenovo g550 as a monitor for mac mini?

Sorry but what you are attempting is impossible. Apple designed the current MM to use an external display not use another computer as an external display unless the computer had Thunderbolt input such as a 2011 or later iMac and the "other computer" also has video input via it's Thunderbolt port.

There are programs like the free (for personal use) TeamViewer that allow you to control one computer from another computer. However, screen updating is slow since the machine being controlled has to generate the screen from the other computer (one with screen) with the CPU vice the GPU) and then send the screen to the viewing computer.