Updating an addon from Gmod 12 to 13?

[serfma]

[I]Before I begin, Yes I am a narb at Lua, and yes I am wanting to learn but from the amount that I know (Which is very sparse) I understand that in order to learn you can not be spoon-fed the answer but I am clueless as to where to start with this.[/I]

I am attempting to fix the ThreeD addon ([url]http://www.garrysmod.org/downloads/?a=view&id=130075[/url]) and update it from Gmod 12 to 13. (I believe it's coded for Gmod 12) However, I am unsure of where to start with repairing it. Any help would be appreciated. I know that I may get flamed in case this is the wrong section to post this in, or because I'm a general scrub at lua in general.

The error:

[ERROR] addons/threed/lua/weapons/gmod_tool/stools/threed.lua:48: attempt to index global 'threed' (a nil value)
  1. LeftClick - addons/threed/lua/weapons/gmod_tool/stools/threed.lua:48
   2. unknown - gamemodes/sandbox/entities/weapons/gmod_tool/shared.lua:251

The threed.lua:

// ThreeD, a 3d text thingamabob for Garry's Mod
// By Sergiusz Bazañski, a.k.a. q3k.
// This code is released under the do-whatever-you-want-but-don't-be-a-douche license.
// www.q3k.org, 2010

//Modified and Maintained by Ugleh since 1.1

TOOL.Category = "Render"
TOOL.Name = "ThreeD Text"
TOOL.Command = nil
TOOL.ConfigName = ""

TOOL.ClientConVar["text"] = "Some Stupid Text"
TOOL.ClientConVar["size"] = 10
TOOL.ClientConVar["color_r"] = 255
TOOL.ClientConVar["color_g"] = 255
TOOL.ClientConVar["color_b"] = 255
TOOL.ClientConVar["color_a"] = 255
TOOL.ClientConVar["font"] = "ThreeD_Coolvetica"
TOOL.ClientConVar["fadecheck"] = 0
TOOL.ClientConVar["fadedistance"] = 512
function TOOL:GetClientBool( cvar ) return self:GetClientNumber(cvar) != 0 end
if CLIENT then
	language.Add("Tool_threed_name", "ThreeD Text Tool")
	language.Add("Tool_threed_desc", "Places a 3D text at the hit position.")
	language.Add("Tool_threed_0", "Primary: Place text. Secondary: Remove text near hit position.")
end

function TOOL:LeftClick(Trace)
	if CLIENT then return end
	
	local Text = self:GetClientInfo("text")
	local Size = self:GetClientNumber("size")
	local Color_r = self:GetClientNumber("color_r")
	local Color_g = self:GetClientNumber("color_g")
	local Color_b = self:GetClientNumber("color_b")
	local Color_a = self:GetClientNumber("color_a")
	local Font = self:GetClientInfo("font")
	local FadeCheck = self:GetClientBool("fadecheck")
	local FadeDistance = self:GetClientNumber("fadedistance")

	local Position = Trace.HitPos + (Trace.HitNormal * 2)
	local Angles = Trace.HitNormal:Angle()
	
	Angles:RotateAroundAxis(Angles:Forward(), 90);
	Angles:RotateAroundAxis(Angles:Right(), 270);
	
	threed.AddText(Text, Position, Angles, Size, Color(Color_r, Color_g, Color_b, Color_a), Font, self:GetOwner():SteamID(), FadeCheck, FadeDistance)
	
	return true
end

function TOOL:RightClick(Trace)
	if CLIENT then return end
	
	local Position = Trace.HitPos
	
	for Index, Item in pairs(threed.Stored) do
		if Item.Position:Distance(Position) < 200 then
			threed.RemoveText(Index)
			return true
		end
	end
	
	return false
end


function RemoveAllTexts(ply, command, arguments)
endint = table.Count(threed.Stored)
for i=endint,1, -1 do
if threed.Stored[i].CreatorSteamID == ply:SteamID() then
threed.RemoveText(i)
end
end
end
function RemoveLastTexts(ply, command, arguments)
endint = table.Count(threed.Stored)
for i=endint,1, -1 do
if threed.Stored[i].CreatorSteamID == ply:SteamID() then
threed.RemoveText(i)
i = 0
break
end
end
end
concommand.Add("threed_remove_all", RemoveAllTexts)
concommand.Add("threed_remove_last", RemoveLastTexts)



function TOOL.BuildCPanel(Panel)
	Panel:AddControl("Header", {Text = "ThreeD Text", Description = "Allows you to place 3D text in-game"})
	
	Panel:AddControl("TextBox", {
		Label = "Text",
		Command = "threed_text",
		MaxLength = "200"
	})
	
	Panel:AddControl("ComboBox", {
		Label = "Select a Font",
		Options = {
			["Akbar"]	= { threed_font = "ThreeD_Akbar" },
			["Arial"]	= { threed_font = "ThreeD_Arial" },
			["Arial Black"]	= { threed_font = "ThreeD_ArialBlack" },
			["Comic Sans MS"]	= { threed_font = "ThreeD_ComicSans" },
			["Coolvetica"]	= { threed_font = "ThreeD_Coolvetica" },
			["Courier New"]	= { threed_font = "ThreeD_CourierNew" },
			["Georgia"]	= { threed_font = "ThreeD_Georgia" },
			["Impact"]	= { threed_font = "ThreeD_Impact" },
			["Times New Roman"]	= { threed_font = "ThreeD_Times" },
			["Typenoksidi"]	= { threed_font = "ThreeD_Typenoksidi" },
			
			["TF2 Build"]	= { threed_font = "ThreeD_TF2Build" },
			["BalladeShadow"]	= { threed_font = "ThreeD_BalladeShadow" },
			["Balthazae Regular"]	= { threed_font = "ThreeD_BalthazaeRegular" },
			["Baloney"]	= { threed_font = "ThreeD_Baloney" },
			["French Grotesque"]	= { threed_font = "ThreeD_FrenchGrotesque" },
			["Babe Bamboo"]	= { threed_font = "ThreeD_BabeBamboo" },
			["Showcard Gothic"]	= { threed_font = "ThreeD_ShowcardGothic" },
			["TF2 Secondary"]	= { threed_font = "ThreeD_TF2Secondary" },
			["TF2"]	= { threed_font = "ThreeD_TF2" },
			["3 theHard way RMX"]	= { threed_font = "ThreeD_3theHardwayRMX" },
			["Coneria Script"]	= { threed_font = "ThreeD_ConeriaScript" }
			}
		})
		
	Panel:AddControl("Color", {
	    Label = "Color",
	    Red = "threed_color_r",
	    Blue = "threed_color_b",
	    Green = "threed_color_g",
	    Alpha = "threed_color_a",
	    ShowHSV = 0,
	    ShowRGB = 1,
	    Multiplier = 255
	})
	
	Panel:AddControl("Slider", {
		Label = "Text Size",
		Command = "threed_size",
		Type = "Float",
		Min = 1,
		Max = 50
	})
Panel:AddControl("CheckBox", {Label = "Should the text Fade into the Distance? (And if so:)", Command = "threed_fadecheck"})
	Panel:AddControl("Slider", {
		Label = "Fade Distance",
		Command = "threed_fadedistance",
		Type = "Integer",
		Min = 200,
		Max = 5000
	})
	Panel:AddControl("Button", {
		Label = "Remove My Last Text",
		Command = "threed_remove_last"
		})
	Panel:AddControl("Button", {
		Label = "Remove All of My 3D Text's",
		Command = "threed_remove_all"
		})
end

[rejax]

[URL=https://docs.google.com/document/d/1khSuIYrAMkqXu7wlH5YRJNwz6hOH6Xqi5lqBhE3x6gA/edit]Extremely useful[/URL]

Other than that, if I'm trying to fix an addon I just follow the error messages and try to fix or replace it as they rise