You know あげる and くれる, right?
They are basically to give and to be given.
So,
私も一緒にする = We will do it together.
私も一緒にしてあげる = We will do it together, so thank me.
本当にするの? = Are you really going to do it?
本当にしてくれるの? = Are you really going to do it for me? That's very kind of you.
And Arnold is usually spelled "アーノルド".