HDMI - How to have both laptop and tv on at the same time?

I just bought a new HDMI cable to reach across the room from my tv to my laptop for watching online shows, youtube, etc. The picture and sound both work perfectly on my tv but the problem is my laptop monitor turns off. I'd like to have both the tv and my monitor on at the same time with the sound on the tv (like with a projector) but I don't know how to configure this. Any suggestions?