how do i make a gui appear to my head then when i touch another player, it moves to his head?

Assuming the gui is in a player's character-

local Players = game:GetService("Players")
local Gui = script.Parent
local GuiOwner = Gui.Parent

local Torso = GuiOwner.Torso

wait(.5) -- Waits half a second so that the person can't tag back instantly.

Torso.Touched:Connect(function(hit) -- Happens when the torso gets touched.
    if hit.Parent:FindFirstChild("Humanoid") then -- Checks if what the player touched has a humanoid.
        if Players:FindFirstChild(hit.Parent.Name) then -- Checks if it's a player.
            local NewGui = Gui:Clone() -- Clones the tag.
            NewGui.Parent = hit.Parent -- Parents the new tag to the touched player.
            NewGui.Adornee = hit.Parent.Head -- Puts the tag above the touched player's head.
            Gui:Destroy() -- Destroys the tag above the old player.
        end
    end
end)

when i join game, the gui doesn't appear over my head, trying to get it where who ever joins game first is it lol.

local rs = game:GetService("ReplicatedStorage")
local Players = game:GetService("Players")
local Tagger = false
local Gui = rs:WaitForChild("Gui") -- Or where ever you have the gui

Players.PlayerAdded:Connect(function(plr)
    if Tagger == false then
        Tagger = true -- So it only happens for the first player
        repeat
            wait()
        until plr.Character.Head -- Waits until the player's character is spawned and the head is found.
        Gui.Parent = plr.Character
        Gui.Adornee = plr.Character.Head
    end
end)

Haven't tested this out but it should work.