How to create a simple multiplayer game?

To expand a bit, I’d like a tutorial on syncing things on the level across clients. I can get multiple clients to connect, but right now I have a countdown that begins at in the level BP BeginPlay, which leads to the unfortunate side effect that when the first client/server starts up, the countdown begins, and when the next client connects their countdown starts. I’d like to somehow sync these up (as in a race game, where we need all clients to begin racing at the same time!)