Other slang meanings for "to go down" "What's going down?" -- A greeting; can be rhetorical, can be an inquiry how the other person is doing, what's happening in their life, whether they know anything interesting. "[Person] is going down." -- Being caught for wrongdoing. Or to fail/lose at something. Could be a threat: "You're going down!" "Going down on somebody" -- Perform oral sex on somebody.

Yes, that is the meaning, it would originally have been used in a criminal environment, but now it just means something is about to happen or start, usually a party or a meeting of friends

Yes, you guessed right. It's slang. And it only means that in this exact construct. I'll write another answer with more information on the other meanings, since you're in a hurry.