How can i script tow truck's hook?

Just insert a weld and have an Touched event,

script.Parent.Touched:connect(function(car)
local w = Instance.new("Weld", script.Parent) --Insert a weld with the parent of script.Parent
w.Part0 = script.Parent
w.Part1 = car
end)

Make it look something like this. This isn't a request site though. This is just to get you started.

This is my very detailed version of LordDragonZord's answer. It's also a little more complete and complex.

names = {"Back Bumper"} -- Put whatever names of the parts you want the hook to hook to here.
hook = script.Parent -- Whatever you want to hook the car to here.

hook.Touched:connect(function(hit) -- Detects when the hook is touched and does something.
    for i,name in pairs(names) do -- This will loop for every name there is and assign that name "name".
        if hit.Name == name then -- Detects if the hit's name is one of the names in the "names" table.
            local weld = Instance.new("Weld", hook) -- Create a weld in the hook.
            weld.Part0 = hook -- Set the first part as the hook.
            weld.Part1 = hit -- Set the seconds part to the touched part wich has one of the names you wanted.
        end
    end
end)