。
I'm going to make him an important offer soon.
Literal
I [topic-は] from-now-on him [to-に] serious offer [object-を] do [intention-つもり] is.
これから means 'from now (forward),' indicating an action about to happen or a future starting now. 申し出をする is the noun-form of 申し出る ('to offer, to propose'). 重大 ('grave, serious, momentous') is a stronger word than 大事 — used for matters with weight or consequence.