How do i hook up my computer to my tv?

ok so i have a Windows Vista Dell Inspiron 1525 and i have a HDTV/LCD Vizio tv. I recently purchased a HDMI cable to hook my PC up to my TV and my computer doesnt even realize that its there. I have tried everything to switch it. I have been to Properties and done the screens and extended the screens. I Have hit FN and F8 and it still wont go. I have actually gotten it to do it for one time... then im not sure what happened. But i do know my computer supports this. Can anyone help? Please tell me how to hook this up.