。
Once I'm done with dinner, I'll head out.
Literal
Dinner [object-を] finish-[conditional-たら] go-out-[sentence-final-よ].
済ませる ('to finish, settle, take care of') is a versatile verb for wrapping up a task or a formality — here, wrapping up the meal. The ~たら conditional gives 'once X happens, Y.' Paired with the future-implying plain form 出かける and the casual よ, this is natural spoken planning.