That's a very very broad question, and not an easy one to answer. It's like asking "how do I make a good film?"
If I'm being honest, there's a limit to how interesting a gaming movie can be, and a limit to what you can do, because obviously you can't choreograph the scenes. As tempting as it might be, don't try to dazzle people with as many different special effects as you can throw at it, but make sure you have a massively thumping soundtrack... the more obscure the better really. And syncing is important too in something like this. Try to keep the scenes as short as possible, people have a very short attention span. Don't linger on any specific scene. Personally i'd try to keep it short and snappy, under a minute maybe.
Also, if you have any in game audio commentary recorded, occasionally putting bursts of that over the top of the music would work, especially if it's an excited sounding person
Start with your second most impressive scene, and end with your most impressive scene.
Finally, all I can say is have a look at videos that other people have done, and do what the good ones have done, and avoid what the crap ones have done
