Now = (1) at the present time or moment: ‘Everything is fine now.’ (2) without delay or hesitation; immediately: ‘Come here now.’ (3) in these times: ‘We now rarely see horse-drawn vehicles on city streets.’ (4) very recently; in the immediate past: ‘He left the office just now.’ (5) used to introduce a command, reproof or request: ‘Now hear this!’ (6) indicates a change of subject or activity: ‘Now the next task is the execution of the proposed plan.’
From now on (WITH on): ‘From now on I’m going to work really hard and make sure I pass.’
Nowadays (WITHOUT in): ‘More women have executive jobs nowadays, especially in publishing.’ ‘Nowadays, computer skills are essential.’
Nowadays is an adverb (NOT an adjective): ‘Video cameras are very popular nowadays.’