Shooting Video with Nikon D5100 and Sigma DC 18-200mm 1:3.5-6.3 HSM?

Hi i'm looking to shoot video with my nikon camera, and sigma lens. But i'm struggling to focus the camera it doesn't seem to focus at all when filming. And the focus is always red.could this be because of my lens? I also try to take photos in live view… And they never focus and again the focus is red… Does anyone know the settings that will work? I have been searching but most say what i already doing.

Added (1). Is it the camera the problem or the lens?

AF won't work on video with this model, you'll just have to get the hang of focussing manually.

You never want auto focus on video… It doesn't work, it's a gimmick for people who are gullible enough to believe it works and you should be doing all manual focus… The only way to shoot video.

Trust me, I did a career in broadcast television video production… Manual focus works if you practice and learn how to operate your lens.