Opponent plays Fizz Trickster. I want to play Disposal Order. When can i response? When opponents plays Fizz or when he choose card to play from trash? And what next? I play Disposal Order an he chooses another card?

You respond to Fizz's "When you play me" trigger — after he's resolved onto the board and your opponent has declared their target spell. And no, they can't swap to a different card afterward.

Here's the sequence:

1. Fizz is played, finalizes, and enters play.
2. His WYPM ability triggers and goes on the Chain — this is when the opponent chooses the spell from their trash (targeting happens as the trigger is placed on the Chain, Rule 352.4.b; FAQ #10020).
3. The State is Closed, but Disposal Order has Reaction, so you now play it targeting that same spell.
4. Disposal Order resolves first (Chain is LIFO): the targeted spell gets recycled to the bottom of their deck.
5. Fizz's ability resolves: its target is gone, so the instruction to play it is ignored — no spell gets played at all.

The opponent doesn't get to pick another card — the target was locked in when the trigger was placed on the Chain, so if it's recycled, Fizz's ability simply fizzles. (FAQ #11060)