First you will need to create a playlist, with some content to DJ with. Once you have at least 1 playlist with at least 1 media in it, click the button besides the current DJ that says "Click to DJ". If someone is already DJing, the button will say "Join Wait List" and clicking it will add you to the queue.
Once it is your turn to DJ, your avatar will move behind the DJ booth and the first item in your active playlist will being playing for everyone else in the community. After your media ends, the next person's turn will begin. The media you just played will now be moved to the bottom of your active playlist.
Tip: If it is your first time in a community, it is often a good idea to sit back and listen for a while first to get a feel for the community. Once you feel the vibe, go ahead and try and pick a song/video that you think the rest of the community will enjoy.