。
I'll make you happy.
Literal
I [subject-が] you [object-を] happy-[make-にする] [insistence-よ].
Notable use of が instead of は — 僕が emphasizes 'it is I who will' (exhaustive listing), contrasting with the neutral topic-marking は. ~にする ('to make into') turns the な-adjective 幸せ into a causative change of state. Sentence-final よ adds assertion.