I've been trying to find how to do it but there's no tutorials on this. I'm trying to make it so when the player joins and after 7 seconds gets kicked
This is really simple, you should've done some more research before asking... anyways I will still help you.
game.Players.PlayerAdded:Connect(function(player) wait(7) player:Kick() end)
Make sure you put that into a local script in replicated first.
I would recommend learning how servers work and go to the ROBLOX Developer Site to learn more about scripting. It's helped me a bit and I think it'll help you on your quest. You also didn't provide some type of coding attempt so the likely of people answering this is slim. Apologies that I can't help much more than with this.