How do I set up a second monitor for my Macbook pro?

I have this issue as well - trying to connect my mid-2014 15" MacBook Pro with Retina, running Yosemite 10.10.5, to a Sony HD TV monitor via HDMI cable - nothing shows up on the Sony monitor via either HDMI input on TV. Everything on my MacBook Pro screen goes haywire when I plug the HDMI cable in (windows/images break up and I can't type anything). Warning on bottom of Sony monitor says it's an invalid input. ????

thanks!

OS X Yosemite: Use your TV as a display

"Connect your Mac to your TV

Connect the video cable (and adapter, if necessary) from the computer’s Thunderbolt port , HDMI port, or DVI port to the TV’s video input port.

Thanks - however, I read and did this already before I came here for help.