I don't want any player to start playing the game before my content is loaded. In other words, I want something like
LoadMyContent() --wait to finish loading, can take 5 - 10s spawnPlayer()`
If you mean the player's character, you could turn off 'Players.CharacterAutoLoad (Wiki article), and call Player:LoadCharacter()
(which must be called server-side) when your game is ready to spawn the player.