here a morph script that i found in freemodels which I just updated for my use

Where morphpart is wherever the part of the model you want to attach to the body and character[bodypart] is where you want to attach the model, just set a variable called "bodypart" to whatever part of the body you want it to attach.

just attach this to a key using UserInputService instead of keydown
http://wiki.roblox.com/?title=API:Class/UserInputService

if you cant make sense of this, youre probably better off learning more scripting for now instead of tackling this.

MORPH SCRIPT =========================================================================

		local morphpart = game.ServerStorage["Equipped Items"][category][item][slot]:Clone()
		morphpart.Parent = character
		local C = morphpart:GetChildren()
		for i=1, #C do
			if C[i].className == "Part" or "Union" then
				local W = Instance.new("Weld")
				W.Part0 = morphpart.Middle
				W.Part1 = C[i]
				local CJ = CFrame.new(morphpart.Middle.Position)
				local C0 = morphpart.Middle.CFrame:inverse()*CJ
				local C1 = C[i].CFrame:inverse()*CJ
				W.C0 = C0
				W.C1 = C1
				W.Parent = morphpart.Middle
			end
				local Y = Instance.new("Weld")
				Y.Part0 = character[bodypart]
				Y.Part1 = morphpart.Middle
				Y.C0 = CFrame.new(0, 0, 0)
				Y.Parent = morphpart.Middle
		end

		local h = morphpart:GetChildren()
		for i = 1, # h do
			h[i].Anchored = false
			h[i].CanCollide = false
		end