You need a reference to the target actor first, depending on the actor type there are many ways to do this but if its a player controller character you can use the way uberman, (good name btw), suggested, “use a “get player controller” node and then from that drag out a “get controlled pawn” node”, then cast to the specific blueprint type that pawn uses, (casting is described well in the official ue4 tutorial series, look those up for more info on casting). Doing this gets you a reference to it, after that you plug that actor reference into a “apply damage” node. After that if your damage is correctly set up in that actor it should do what you want.
//hf gl