!ask if I have an astral heron at base and play my first card and then go to the battlefield my second card will have the heron discount or I need to play my first card while heron is on the battlefield?

No — the first card must be played while Heron is already at a battlefield.

The "if I'm at a battlefield" clause is part of the trigger condition, and it's evaluated at the moment your first card is played (Rule 383.2.a.1). If Heron is at your Base then, the condition isn't met, so no trigger ever fires — moving him to a battlefield afterward doesn't retroactively create the discount. This is the mirror of the death/bounce rulings: once the trigger has fired while Heron was at a battlefield, later removal doesn't cancel it — but if it never fired, there's nothing to save. Play Heron to a battlefield before playing your first card (or move him first) and the discount applies to your second card.