its my opponent turn. A combat showdown starts. Im the attacker. Who plays first?

Yes — you play first. The attacker gets Focus (and priority) the moment the Combat Showdown begins, regardless of whose turn it is; the turn player only matters when it's not a showdown (Rule 466.2.c.1.a, 345).

So the order is: attacker establishes as Attacker, gains Focus, then gets the first chance to play an Action, activate an ability, or pass — and if you pass, Focus goes to the defender.

One wrinkle: if attack/defend triggered abilities are pending, an initial chain forms and you place your triggers first, and Focus does not pass when that initial chain resolves — you still act first afterward (Rule 466.2.d.1, 346.1).