As was stated, Machinima, (machine-ima) is any movie that's created using a video game. So basically, you're recording the game as you play it, and you use that to make a movie. Whether you mod the game, and show the differences, use in game conematics or footage you take to make a story-based movie, or mash the cinematics of the movie together for humorous effect, is entirely up to you.
You don't need to know how to model, or how to MPQ edit. The only thing you need to know, is how to screen capture. And there is a simple, powerful, very popular and I-promise-spyware-free program that everyone and their brother uses.
Fraps. You can download it at
www.fraps.com. It is very easy to work.
Warning: FRAPS records your screen in AVI format at a VERY LOW compression rate. Clips that are 30 seconds long could be OVER 500 MEGABYTES in size, so make sure that you have harddrive space available.
If that doesn't work out, you could also try GameCam, another popular one that works in much the same way. I don't like it as much, but my friends do.
Windows Movie Maker is the tool you need. Don't bother with Premier or anything fancy, as they cost too much money, and Movie Maker is free, comes with Windows, much easier to use and still powerful.
Have fun!
W