。
I'll be your witness.
Literal
I [subject-が] witness [become-に-なって] give [emphasis-よ].
~てあげる is the giving-receiving verb for 'do for [someone else's] benefit' — the speaker does the action to benefit the listener. 証人になる ('to become a witness') uses になる for the change of state. A chivalrous offer of support, common in legal or moral testimony contexts.