The initiator is the one who starts it, whatever it is. The guy who shouts "Let's get this party started!" is the initiator.
An initiator is an instigator, the one who begins something. Initiator comes from the Latin initiare, meaning “beginning.” The spark that lights the fuse is the initiator of the bomb’s explosion. John F. Kennedy was the initiator of the US Peace Corps. Hockey players are expert initiators of fights that, if happening on the street, would often result in the arrests of the participants.