Hello How do i use a variable in another variable to adress something?

To answer your question, you need to wrap your variable in [ ]. Here is the new code.

local text = game.Workspace.Health_Teller.BillboardGui.health
game.Players.PlayerAdded:connect(function(player)
    player.CharacterAdded:connect(function(character)
        local player_name = player.Name
        print(player_name)
        local player_hum = game.Workspace[player_name].Humanoid
        print(player_hum.Health)
    end)
end)

But there is a better way to do this. The CharacterAdded event of Player passes a reference to the character (That's what the character argument in function(character) is). The code that utilizes the character argument is below.

local text = game.Workspace.Health_Teller.BillboardGui.health
game.Players.PlayerAdded:connect(function(player)
        player.CharacterAdded:connect(function(character)
        print(player.Name)
    print(character:WaitForChild("Humanoid").Health)
    end)
end)

I hope my answer could solve your problem! If it did please remember to mark it as correct!

local text = game.Workspace.Health_Teller.BillboardGui.health   
game.Players.PlayerAdded:connect(function(player)
-- you dont need to add character added just say player.Character
local player_name = player.Name
print(player_name)
local player_hum = game.Workspace:WaitForChild(player_name).Humanoid -- fixed
print(player_hum.Health)
--removed one end because you can just say player.Character
    end)