How do I make game rounds start with players that are in the lobby?

Ok so what I'm having trouble with is this. I'm just now learning how to program, but I want to be able to make a script that launches a round. I want it to clone the map, and teleport all the players that re in the lobby, in a random position of the map. And not only that, but have a GUI launch when the players are in the game, not the lobby. So like, the gui will only be visible if youre in the game, not the lobby. But the thing is, i watched so many videos but still confused on how to make it.