How do i make a body part do damage for a Combat script?

You can use the Touched event to determine if a player is touched by the body part. Manipulate their Humanoid object's Health property, then.

local person = workspace.Goulstem.HumanoidRootPart --Part to affect
local damage = 5 --How much it damages
local db = false --Debounce

person.Touched:Connect(function(hit) --Touched event 'hit' is what touched
    --make sure it's not yourself
    if hit:IsDescendantOf(person.Parent) then return end
    if not db then
        db = true
        local h = hit.Parent:FindFirstChild("Humanoid") --Check for humanoid
        if h then
            h:TakeDamage(damage) --Damage player
        end
        db = false
    end
end)