Why is this on a wiki? Using a wiki to host a stykz collab may seem a little weird, but by doing it this way the success rate should go up. The Stykz forum has hosted many collabs that have eventually failed, and by veering away from the traditional forum environment this will give the 25 Song Collab a chance to succeed.
Who can edit? Everybody has input and can edit, so the responsibility does not fall on any one person. Animations, and the ideas for those, are public. If someone decides not to finish an animation, somebody else can pick it up and finish it. That is just one of the examples of how using a wiki will be helpful.
Who is going to put all the finished animations and song clips together into the final video? Most likely MPK Animations (EmPeeKay), the founder of the collab. But if he doesn't, it's possible for anybody else to do it.
Where can I get my song and how do I edit it to make it shorter? You can find your song in many places, try using an mp3 search such as mp3raid. To trim your song so it's only the part you want, use a free audio editing tool such as audacity.