How do I make something only appear when I enter the game?

I've been toying with this idea a lot, and was hoping for some feedback. I'm really new still, and have no idea how I would even begin to do this.

If you are trying to do specific thing when you join, you can check the userid of player. For example, this code will spawn a block when you enter.

local myUserId = -1  --Replace it with your userid. Here, I use the Player1's userid. 

local Players = game:GetService("Players")

local part = Instance.new("Part") --thing to insert. it can be model, union, whatever you want.


Players.PlayerAdded:Connect(function(player)
	if player.UserId == myUserId then
		--Write your own function here.Mine is just a example.
		clone = part:Clone()
		clone.Parent = workspace
	end
end

--EDIT-- If you want the event to end if when you leave, you can use the Players.PlayerRemoving function.

Players.PlayerRemoving:Connect(function(player) if player.UserId == myUserId then clone:Destroy() end end