How to make your character as someone?

Well what I did is make a localscript and put it inside the StarterGui because that way the script will automatically run each time the player respawns.

--To modify the player, you may use the following code
game.Players.LocalPlayer.Character.Shirt --or whatever you want to access inside the Character

To give everyone a hat, you would want to clone it into the character. What I did in my script was have a model in the workspace where I stored all the things I want to clone over to the player.

I also used the following code to remove some of the unwanted contents the player spawns with

for a,c in pairs(game.Players.LocalPlayer.Character:GetChildren()) do --Make a list of all the things inside the LocalPlayer's Character. "a" is the index value and "c" is the actual element.
			if (c.className == "Hat") or (c.className == "CharacterMesh") then
			c:Destroy() --Get's rid of the hat or charactermesh
end

Here is an article on pairs loop if needed.

Hope you figure it out.