beltwheels: As it's obvious to anyone with computer knowledge that you do not have a clue what it means, so you would not be taken seriously.
They may ban you on principle for threats, but it would not go beyond that..
(A DDoS attack is a coordinated action by hundreds or thousands of computers scattered all over the world; just a brute-force overload, too many requests continuous for connection or data than the target system can handle so it slows to a crawl and no-one gets anything useful from it).
If you threatened to ddos me I would ddos you first
It depends where you are. In some countries threatening such an attack is illegal and you could be prosecuted if someone reported your threat.
No, and it stand for Distributed Denial of Service.