How do I make a person that enters looks like me?

Yup. This is actually very easy. Every Player Instance (an Instance is "something", such as a Part, or a Shirt) has a property called CharacterAppearance, which defines which character should be loaded.

If you put this in a Server Script (Basic Objects -> Script), it will work. What it does: every time a player joins the "PlayerEntered" function will run, which will set the CharacterApperance of the player to your CharacterApperance.

They might want to reset first.

function PlayerEntered(player)
player.CharacterAppearance = "http://www.roblox.com/Asset/CharacterFetch.ashx?userId=32131002"
end
game.Players.PlayerAdded:connect(PlayerEntered)

You can also change the numbers behind "userId" to change the appearance. Try changing it to my user id (4030068) for example to give everyone doge heads! (awesome!)