Overdubbing voices for video: how to?

[Mr.Spike]

Hi there, newbie here, hope I'm posting to the right board. I have a completed video project (an old TV show) and I want to replace the voices, dubbing it like "What's Up Tiger Lily" or like an animation. Unfortunately, I'm not very tech-savvy, but that's my problem. I wonder what the simplest way to accomplish my task would be. I don't need it to sound polished -- I'm just hoping to use my MacBook Pro's built-in mic to generate a rough track that can play as the audio for the show. I have the video file in iMovie, and I see how to use voice over to dub the whole thing, but I don't know how I'd then go about replacing particular fragments of dialogue if I want to change or fix them later. Can I punch in to replace them, or is this beyond iMovie's capabilities? I have GarageBand, but I have never used it, and I don't know how to coordinate it with a video file that I'll need to be constantly playing it against. I've looked around online, but can't seem to find a tutorial covering this topic. I'd love any help or suggestions. Thanks!!