how to have a decal pop up on character body part when damaged?

This website isn't for giving out free scripts, but here are the building blocks.

you want to detect if the player's health changed here's the article: https://developer.roblox.com/en-us/api-reference/event/Humanoid/HealthChanged

basically, the code from there is (make sure its in a local script or whatever)

local char = game.Players.LocalPlayer.Character
local humanoid = char:WaitForChild("Humanoid")

humanoid.HealthChanged:Connect(function(health)
end)

then you want to Instance a decal onto there

local char = game.Players.LocalPlayer.Character
local humanoid = char:WaitForChild("Humanoid")
local Texture = "http://www.roblox.com/asset/?id=4019275819"

humanoid.HealthChanged:Connect(function(health)
    local decal = Instance.new("Decal",char.UpperTorso) -- if your game is r6 replace UpperTorso with Torso

    decal.Texture = Texture
end)

There you have it, when a player's health is damaged there will be a decal instanced onto it, don't forget to put in a assetid. (Btw, I assumed you were referring to blood instanced on the body so I already put on a blood id onto there, and here's a script if you want it to disappear after any amount of seconds just edit the variable called wait).

local char = game.Players.LocalPlayer.Character
local humanoid = char:WaitForChild("Humanoid")
local Texture = "http://www.roblox.com/asset/?id=4019275819"
local wait = 3

humanoid.HealthChanged:Connect(function(health)
    local decal = Instance.new("Decal",char.UpperTorso) -- if your game is r6 replace UpperTorso with Torso

    decal.Texture = Texture
    wait(wait)
    decal:Destroy()
end)