I would like to auto-tune a video, but I do not know how. Please tell me how I can achieve that effect.
1: Export the audio, you want the audio exclusively, and you want it in lossless format (wav, aiff, flac) (unless your DAW in the next step supports video). You might need a video editing tool for this one.
2: Import the audio to a DAW: protools, live, logic, nuendo, whatever.
3: Apply auto-tune.
4: If your DAW can't deal with video, you need to glue back the video and audio using a video editing tool.
The question is very board as is though, so it's hard to help you further.
Or perhaps there's a video editing tool that supports audio plugins? If this is the case, you just have to load autotune (or other pitch corrector) into the video editing tool and you are done!