How do I make a part damage a player for every second that the player is touching the part?

You can achieve this by using tables and loops:

local PlayersInRadius = {}

local CreatorInRadius = false 

local function runDamageOnPlayer(player)
	while player and PlayersInRadius[player.Name] do 
		wait(1)
		local Character = player.Character
		if Character then 
			local IsInRadius = false 
			local PartsTouching = Character.HumanoidRootPart:GetTouchingParts()
			-- GetTouchParts() can be called on a BasePart. It returns a table of parts that collide or intersect with the given BasePart.
			for i, part in pairs(PartsTouching) do 
				if part == Garden0 then -- match part with 'Garden0'
					IsInRadius = true -- set bool to true
					Character.Humanoid:TakeDamage(10) -- You can use the TakeDamage() function instead. Remember that player's with a force field won't take any damage using this method.
				end
			end
			if not IsInRadius then 
				-- Cancels the loop if 'IsInRadius' bool is false 
				break
			end
		else
			-- Cancels the loop if 'Character' is nil
			break
		end
	end
	-- Note that any code below a loop will not run unless the loop was canceled using 'break' or wrapped in a function (spawn, delay, etc..)
	PlayersInRadius[player.Name] = nil -- Set stored value of the player to nil (similar to table.remove)
end

local function runHealingForCreator()
	while CreatorCharacter and CreatorInRadius do 
		wait(1)
		local IsInRadius = false 
		local PartsTouching = CreatorCharacter.HumanoidRootPart:GetTouchingParts()
		for i, part in pairs(PartsTouching) do 
			if part == Garden0 then 
				IsInRadius = true 
				Character.Humanoid.Health = Character.Humanoid.Health + 10
			end
		end
		if not IsInRadius then 
			break
		end
	end
	CreatorInRadius = false 
end

Garden0.Touched:Connect(function(part)
	local Humanoid = part.Parent:FindFirstChild("Humanoid")
	if Humanoid then 
		local PlayerTouched = game.Players:GetPlayerFromCharacter(part.Parent)
		-- GetPlayerFromCharacter() has one argument which is the player's character model and returns the player
		if PlayerTouched then 
			if not CreatorInRadius and CreatorCharacter == part.Parent then 
				CreatorInRadius = true 
				runHealingForCreator()
				return
			end
			if not PlayersInRadius[PlayerTouched.Name] then 
				PlayersInRadius[PlayerTouched.Name] = PlayerTouched -- Stores the player in table
				runDamageOnPlayer(PlayerTouched)
			end
		end
	end
end)